If I have a column that is 26 rows showing values which are never duplicated
and these values are auto populated from another source data into this
column, is there a way or formula that can tell me by displaying in another
column or row which ones are missing. I know what the possibilities of the
values are and there are only 26. Thx in
advance.

The values:
1
1A
1B
2
2A
2B
3
3A
3B
4 thru and including 20