The error messages should indicate what the problem is. See the error message list in your calculator's user's guide for more information. 
For example:

Ma ERROR or Math ERROR

Cause: The calculation result is outside of the allowable calculation range or you are trying to perform an illegal mathematical operation (such as division by zero). 

Action: Check your input values and adjust them to allow the calculation result to be within the allowable range. If you are using memory for your calculation, make sure that you check the memory value. 

Syn ERROR or Syntax ERROR

Cause: The calculation expression is the wrong syntax, or the program you are using contains a syntax error. 

Action: Press the <right> or <left> key to display the location of the error, and correct it. 

Stk ERROR or Stack ERROR

Cause: The calculation you are performing has caused the capacity of the numeric stack or the command stack to be exceeded. 

Action: Simplify your calculation expression. You could also split your calculation expression into two or more parts and execute the parts separately. 

Mem ERROR 

Cause: You executed a matrix, graphing, or some other operation that caused remaining memory capacity to be exceeded. 

Action: Delete memory data you no longer need. Make sure that the number of variables used in your expression does not exceed the number of variables that are available. 

Other error messages may appear to let you know that a calculation you are trying to perform is not compatible with your current mode, angle unit, display, or other setting. If this happens, check the setup of your calculator and try again. 

Yes. The calculator has a two or three angle modes, and you need to check to make sure that the currently selected angle mode is the correct one for the type of calculation you are trying to perform. If it isn't, you need to change it. 

  Determining the Current Angle mode Setting

The "angle" setting on the calculator's setup screen will show one of the three angle unit indicators described below. 

Degree: 
If "DEG" is on the display, it means that the current angle mode is Degree. 
A degree is 1/360 of the circumference of a circle. One degree is represented as 1゜. 

The following are calculations that use degrees. 
sin 90°= cos 0°=1

Radian:
If "Rad" is on the display, it means that the current angle mode is Radian. 
A radian is 1/2πr of the circumference of a circle. Measurement of angles using radians called the "radian system of angular measurement".  1 radian = 360゜/(2π) 

The following are calculations that use radians. 
sin(π/2)=1  cos (2π)=1

Grad:
If "Gra" is on the display, it means that the current angle mode is Grad.  Note that ClassPad300 does not have a Grad angle mode. 
A grad is 1/400 of the circumference of a circle. 

The following are calculations that use grads. 
sin 100 grads=cos 400 grads=1
The following shows the relationship between the three angle modes. 
90°=π/2 radians=100grads

Radian is the initial default angle mode for a graphic scientific calculator. 

New features:

• High-resolution color display with over 65,000 colors
• Natural V.P.A.M. natural input and output mode (For example, when you calculate using the fx-CG Series, you can input a proper fraction, logarithmic function, integral, derivative and matrix in a mathematical expression in the textbook.)
  Innovative Picture Plot application for photo and video analysis
• Simple to connect to a PC as a USB mass storage device
• Can be connected directly to Casio USB projectors
• The four application software programs have been added in addition to the capabilities of a conventional graphing calculator as follows.
• Conversion 
• Picture Plot
• Geometry
• Physium

When resetting the calculator, is to reset the Setup Data and/or the Main Memory.

The initial default setting of the calculator is MathI/MathO. With this setting, fraction format is given priority for calculation results. ClassWiz lets you change the setting to specify decimal value format for results.

The [MODE] key can be used to select the modes described below.

1) Calculation Modes 
Before performing a calculation, you first need to select the appropriate calculation mode. 
COMP: Basic arithmetic calculations 
(arithmetic operations, trigonometric functions, etc.)
COMPLEX: Complex number calculations 
STAT: Statistical calculations 
BASE-N: Binary, octal, decimal, hexadecimal number system calculations 
MATRIX : Matrix calculations (determinants, etc.)
VECTOR : Vector calculations 
EQN : nth degree equations, simultaneous equations 
TABLE : Generate a number table based on one or two functions

2) MthIO/LineIO Settings 
Specifies the display format.
MthIO-MathO or MthIO displays input and calculation results using the same format as they are written on Paper.
MthIO-LineO displays input the same way as MathO, but calculation results are displayed in linear format.
(ES PLUS Series only)
LineIO causes fractions and other expressions to be displays in a single line.

3) Display Settings 
Use these settings to control how calculation results are displayed. 
Note that changing the Fix and Sci settings does not affect internal calculation results. 
Norm: Exponential display format and cancels the currently configured Fix and Sci settings. 
Fix: Specifies a fixed number of digits to the right of the decimal place. 
Sci: Specifies a fixed number of significant digits.

4) Angle Unit Setting
(90°=π/2 radians=100 grads)
Deg: Degrees
Rad: Radians
Gra: Grads