Unfortunately, the maximum number of nested IF statements in Excel is 64. When you add another level to your formula, it exceeds this limit and you get the error message you mentioned.

One solution to this problem is to use the CHOOSE function instead of nested IF statements. The CHOOSE function allows you to select a value from a list of options based on a numeric index.

Here's an example of how you could use the CHOOSE function to replace your nested IF statements:

1. =CHOOSE(A1,M1,N1,O1,P1,Q1,R1,S1,T1,U1,U2,U3,U4,U5, U6,U7,U8,U9,U10)

In this formula, the first argument (A1) is the numeric index that determines which value to select. The remaining arguments are the list of options to choose from.

This formula is much simpler and easier to read than the nested IF statements, and it doesn't have the same limitation on the number of levels.