Cannot generate the next number for the sequence.

Hi @KenC This error is shown if the “AP/PR Last Reference Number” is not configured properly, for the respective Cash Account of the Payment Method. 
Solution:

IF autonumbering suggestion is required, the provide proper numeric value in the “AP/PR Last Reference Number” field. 

If auto numbering suggestion is NOT required, then uncheck the checkbox “AP/PR Suggest Next Number”.

Screenshot for reference:

Error seen for the above settings:

Error resolved for the below settings for the Cash Account:

Thanks