It is highly recommended to run a description on the variables you intend to use. This will allow you to verify that the selected variables have data. If you do not run a descriptives and use a variable with no data, you will most likely encounter an error such as: 

To run a descriptive, follow the steps below:

Step 1:

A) Click Analyze

B) Select Descriptive Statistics

C) Select Descriptives

Step 2:

Enter the variables you would like to check for data

Step 3:

Click OK

Step 4:

Review the output for any field with a zero

Any field with a '0' indicates that there is no data for that variable.

You may want to change variables or just eliminate the variables with no data.