Trying to insert an invoice category in column B based of the code number in column A

Those are probably doable with vlookups or index/match formulas depending on the details.
if you are wanting to somehow insert both columns based on something then that is likely going to require VBA.
To get better answers, you should mock up a sample workbook WITH representative data AND expected results so we can give better advice.
click go advanced under the quick reply window, then midway down click manage attachments, then at the top >> browse, then upload, close this window and submit reply to upload a workbook.