I am trying to create an IF formula that I can later paste into data
validation. However to keep this question simple at first, I will ask it this
way.

Try this formula in a blank spread sheet, in cell B1:

=IF(A1=1,M1,IF(A1=2,N1,IF(A1=3,O1,IF(A1=4,P1,IF(A1 =5,Q1,IF(A1=6,R1,IF(A1=7,S1,IF(A1=8,T1,0))))))))

That works but if you add another "level":

=IF(A1=1,M1,IF(A1=2,N1,IF(A1=3,O1,IF(A1=4,P1,IF(A1 =5,Q1,IF(A1=6,R1,IF(A1=7,S1,IF(A1=8,T1,IF(A1=9,U1: U10,0)))))))))

....You get the following error:

The specified formula cannot be entered because it uses more levels of
nesting than are allowed in the current file format.

Can we change the file format? Is there another totally different method to
achieve this? Do you have any remedy for this at all?

Thanks for your consideration.