Hi Brain,
For this issue, we have reported it to the related team. Once there is any update, we'll get back to you. To benefit other community members who have the same issue, I'd like to share a summary.
Symptom:
When using Solver on Excel 2016 for Mac, the previously created workbook (the solver was used on that workbook)will open. Solver can work on the newly created workbook and the old workbook can't be closed or modified.
Workaround:
Quit Excel 2016 for Mac and restart it.
Assessment:
Our related team is investigating it.
Regards,
Felix