What am I doing wrong here? (i.e. inserting incremental letter)

[ohrenkino]

If %1 represents the whole filename, then

@cmach1

$num(%1,1)

will always be 0 as an "A" is no number and therefore the numeric value = 0.
see also your post from a couple of days ago:

How to get an incremental letter

You would have to supply the original filename, the pattern you used for the original filename and the pattern for the target filename. If A1 is the %1 part, then $num(%1,1) will always return 0 as value.

To get an incremented letter at the end of the filename of the selected files, do not use Convert>Filename-Filename but Convert>Tag-Filename
Format string: %_filename% - $char($add(64,%_counter%))