Hi CBNBuilding
Welcome to the MYOB Community Forum, I hope you find plenty of useful information.
Thanks for your post. When you tried to create a new super fund, did the drop-down arrow grey out? You may add a letter to the Superannuation Fund Name like what I provided on the screenshot below as a sample, then put in all the correct details of the updated superannuation fund. This will allow you to save the updated and have it selected under the employee card file by clicking on the dropdown and locating the updated one we just saved.

After clicking OK at the upper screenshot sample, you may select the updated one by clicking on the arrow down beside the Superannuation fund tab. You may refer to the another screenshot below:

This steps will let you change or update the Superannuation of your employee without getting the same error message. You may refer to this Help Article:
Set up superannuation funds as it will provide you with detailed steps on how to create a new super fund.
If you need further don't hesitate to start a new post and we'll be happy to assist.
If my response has answered your enquiry, please click "Accept as Solution" to assist other users find this information.
Cheers,
Shella_A