Null error message of import excel spec

Hello

The study I'm working at had already done define last year. We're updating the define using the updated dataset during this year. I exported the Excel Spec from last year and did a lot of updated using this xlsx file, so the format should be correct. I was about to import this to update the define on the enterprise, but it kept showing null error message. I have no clue how to solve this error. I've checked if there's empty rows in the spec or if I opened the file at the same time or maybe the variable format is different(character or number), but non of them solved the problem. Does anyone on the forum know how to solve a null error message?