awk does not come with a built in debug option.

I have come across but never used awkdb:

AWK Debugger

Usually I just print out various messages or values of fields
to see what is going on.

If you are using gawk, you could recompile the source with
the debugging option turned on to print out the parse stack
information as gawk executes - but this will probably give
you far more information than you can use.