Step 1: Run excel as administrator and check PI Builder
Step 2: If not fixed, Repair PI Builder
Step 3: If not resolved, repair Office 365
Additional reference :
This all boils down to OSIsoft's registry implementation for Excel-addin's,
The way OSIsoft get's it's known servers tables via the registry isn't supported by Microsoft in Office 365 and this issue has been comming and going since 2014. OSIsoft is working on a new way of storing this so hopefully in the next version we will have a permanent fix on this.
As mentioned above, repair on the PI Builder (or Data Link) installation should resolve the issue.