auto increment based on last value used in Excel table.

hammer54man 

You could do a lot to help us help you by giving a few more specifics. Ideally, unless your check book register is highly confidential--granted that it's personal, but that needn't mean you need to keep it secret, unless there are expenditures, payees or amounts that you'd rather not be showing publicly--ideally, you could help by posting a copy of the spreadsheet on OneDrive or GoogleDrive with a link pasted here that grants access to it.

Short of that, since there are quite a few error messages that Excel could show, tell us what the specific error message is. 

But realistically, your question as it stands leaves a lot of questions unanswered. What data (including check numbers) are you capturing, when did this error start appearing, how long had you been using it before, does it happen all the time now, is there anything unique about the numbers over which you're getting the error message? And so on? 

To diagnose a problem, one needs a more complete description of the symptoms, or the chance to examine the patient first hand. The latter would be the most helpful.