Warning instead of #N/A

[BarnBrian]

Is there any way that when I have Lookup in a range of cells and #N/A appears that insead of #N/A an instruction appears.

 

[Peter_SSs]

Is the error occurring because
a) the lookup value cell is empty?
b) the lookup value cell is not empty but the lookup value does not appear in the lookup range?
c) the cell that gets looked up in the lookup range already contains the #N/A error?

Here is one way that should work in all of the above cases, but there may be a simpler way, depending on the answer above:
=IF(ISNA(your current lookup formula),"instruction",your current lookup formula)

 

[BarnBrian]

Option B is the situation

 

[Peter_SSs]

Option B is the situation

You haven't provided any details of ranges, how your sheet is operated etc, so let's suppose your current formula is ...
=VLOOKUP(A1,$C$2:$D$10,2,0)
... then a few options might be:

1. The formula already given above, which would become:
=IF(ISNA(VLOOKUP(A1,$C$2:$D$10,2,0)),"instruction",VLOOKUP(A1,$C$2:$D$10,2,0))
2. =IF(ISNUMBER(MATCH(A1,$C$2:$C$10)),VLOOKUP(A1,$C$2:$D$10,2,0),"instruction")
3. Use Data Validation in A1 to stop the user entering an entry that is not in C2:C10 in the first place.