Yes, I am having this issue too.  Started last week. It has happened before, and I don't recall how it was resolved.  I am searching for a solution but not having much luck!

The part number is critical to id the specific items within the inventory module. Sage does a search and if the number is already in use you will not be able to duplicate it. See if you can find the part number already in use and modify it or delete it so you can add the new data for that number.

Always do a back up first before making this change as it may cause problems with history if the part was used in the past.

The easiest way would be to modify the part number.

The new items that I am trying to create are NOT duplicates.  I initiated a support chat, was given a couple things to try, the first suggestion did not resolve the issue.  Haven't completed the second suggested action, as we've had customers in store - will be trying later. 

Hi Patty: I hope the suggestions you got from Chat work for you.

The support chat suggested action worked for me!

"Open your company file in single-user mode with the sysadmin user. Go to Setup / Setup Users. Enter the sysadmin password. Take a screenshot of the user’s setup screen (because you will have to recreate these users). One by one, select each user and click Remove User, except the sysadmin. When only the sysadmin user is left, select it and click Modify User. Remove the sysadmin password: enter a character in the Enter Password box, press the Backspace key, and click OK. Recreate the usernames and passwords for all users. Add the email addresses if this is a file shared with RDA"

The step that says to remove the sysadmin password... the system wouldn't allow me to do this. I only have one employee, so it was quick and easy to remove and recreate one user, and that worked for me, I am able to create inventory items again!