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I can't duplicate the issue you are describing. Whether A3 and A4 are 0's or blank, I still get the result "Excess"
I even tried entering the A2 data as text and entering text into A3 and A4. It did disrupt the formula results but did not result in an #N/A.
First thing would be to check the formula carefully to be sure it is still referring to the correct cells and have the correct compare symbols. I am cautious to offer changes to the formula without understanding why it is getting the error. The only thing I could do is to add an IFERROR component to the formula, but without knowing the cause, I don't know what other effects that might have. You can try this and see if it works better without any other ill effects.
=IFERROR(IFS(AND(A2>=A3,A2<=A4),"In Range",A2<A3,"Low",A2>A4,"Excess"),"Excess")
The last "Excess" could be changed to anything inside the quotes if that ends up being a better option.
EDIT:
I did get the #N/A if the compare symbol in the last condition is wrong, either < or =. That results in the formula not finding any TRUE condition which is when the #N/A occurs so check those symbols very carefully.