Sage also suggests deleting the suspect invoice to resolve. (Read HERE.) However, you can try re-indexing the data file from Sage File maintenance:
You can also confirm if the Lock Autonumber option enabled under Settings > Invoice & Order Defaults:
Enabling that option typically prevents the UI from inputting an invoice number. Disabling that option allows input of an invoice number on the UI, and Sage will increment the next invoice from last used number.