First 4 CURP positions must be alphabetical "&"

What causes this issue?

You have entered numbers or blanks in the Population Register Single
Code instead of the required alphabetical characters .
The first four CURP characters must be letters and must not have any
blanks in between.

System Response

The system interrupts the program run.

How to fix this error?

Make the corresponding changes in order to correct the CURP.

Error message extract from SAP system. Copyright SAP SE.