Hi
Base on the code you show, the only possible error I see is a missing comma. While this will not produce the error you describe, it will be the next available error.

The proper code should read:

UG,100,48\AX-,4\AP,4,P,Z,P,/P\0.020

Is it possible you are taking the first three points in a straight line? The software will not be able to determine the planes axis direction which is a requirement.

Simply try off setting one of the first three points. Other wise you may be required to stagger the save points ex.

1 2 3
4 5 6
7 8 9

change one reads
1 5 3
4 2 6
7 8 9

change two reads
1 6 3
4 9 8
7 2 5