One way is to have all the values in one Col say Col E and enter this formula
against the value in E1 in cell F1
=VLookup(E1,A:A,1,False) assuming the values which you are checking are in
Col A
copy this down to F26
The entries against which you get #N/A are the ones missing from Col A...

"Shu of AZ" wrote:

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