Hi Sarah,
As the error message indicate, could you go to Finder > Application > Right click on Excel app > Show Package Content.
Then go to Contents > Resources > en.lporj > Add-Ins
See if Solver.xlam is located in this folder or not.
If solver.xlam does exist in that destination. Please close your Excel app, click Go on the top ribbon of Finder > Go to Folder> input the following path:
~/Library/Containers
Under this folder, search for a folder named as "com.microsoft.excel" or "Microsoft.Excel", move that folder into trash.
Open Excel again and try load the solver add-in under the Data tab.
Regards,
Alex Chen