Dear snube chou,
Good day! Thank you for posting to Microsoft Community. I am happy to help you.
I have recreated the sample on my side, and am just wondering if "When filling in A3, the formula will report an error." means incorrect value returned in cell C1. I assume this part LEN(A3=0) in the formula is written improperly based on the other two logic tests for A1 & A2.
If this is the case, I can't think of a possible workaround to skip the validation. I would recommend you reach to the author of this file or anyone has edit permission for the fix.
Sincerely,
Ryan | Microsoft Community Moderator
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