unable to resolve a #NUM! error message

Welcome to the board.

Post an example workbook.

What answer do you expect in those that are returning NUM? My first guess would be that something in the inputs in those cases causes the internal algorithm for the NPER() function to fail, and this error is Excel's way of saying that this combination of inputs does not yield an answer. If you can enter the same inputs into an alternative calculator and get a reasonable answer, then you may need to consider alternatives to the NPER() function. If you enter the same inputs into an alternative calculator and it returns an error as well, that would suggest that that set of inputs does not have a solution.

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I am not financial analyst, but that sounds to me like the inputs were bad for those incidences of the function.

Does that "resolve" the NUM error for you then, or is there still more to your question?