Missing numbers in a column of repeating numbers

Katescott Now it is more clear, you can use the following formula:

=LET(A, A1:A9, B,B1:B9, check,SEQUENCE(3), ux,UNIQUE(A),
 missing,MAP(ux, LAMBDA(x, TEXTJOIN(",",,FILTER(check, 
 ISNA(XMATCH(check,FILTER(B,A=x))),"")))),
 FILTER(HSTACK(ux,missing),missing<>""))

In this sample, we assume the missing values we want to check are in the range of 1-3. Here is the sample output. It returns the rows from the first column with missing values. If more than one is missing, it returns a list separated by a comma. Here is the output, you can adjust to your need in your real situation, adjusting the names: A, B, and check. The last FILTER call is to just show the column row values with missing values only.

Here is the output:

You can also use COUNTIFS as follows:

=LET(A,A1:A9, B,B1:B9, ux,UNIQUE(A), seq,SEQUENCE(,3), 
 cnts,COUNTIFS(A,ux,B,seq), missing,BYROW(cnts,LAMBDA(x,
 TEXTJOIN(",",,FILTER(seq,x=0,"")))), 
 FILTER(HSTACK(ux,missing),missing<>""))

In both cases you can use MAP or BYROW I just changed it for illustrative purposes. For this case, seq has to be a column-wise array.

If you want the result the other way around, i.e. for each number the missing letters, adapting for example the COUNTIFS approach and considering a numeric range or 1-5:

=LET(A,A1:A9, B,B1:B9, ux,UNIQUE(A), seq,SEQUENCE(,5),
 cnts,COUNTIFS(A,ux,B,seq), missing, TOCOL(BYCOL(cnts,LAMBDA(x,
 TEXTJOIN(",",,FILTER(ux,x=0,""))))),
 FILTER(HSTACK(TOCOL(seq),missing),missing<>""))

Now we use BYCOL and transpose the information using TOCOL.

I hope it helps,