It means you are using a variable incorrectly. For example, when you make a variable, say X, it can be a number, but not a text string. If you try to do X="TEST", you get this error. If you want to make a variable hold a string, you need to put a "$" at the end of the variable name, like this : X$=TEST .
You also might be putting text as an argument in a command, like SPSET 3,"WA" would...
The value your code is trying to assign to a variable doesn't match the variable's "type". Variables types are numeric, string, array, etc.
These are valid
A$="hello"
A=42
B$=A$
B=A
These are invalid
A$=73
A="bye bye"
B$=A
B=A$
