Excel add-ins tab

My Add-ins tab keeps appearing and disappearing - mostly disappearing.  I have PKSolver as an Add-in and cannot access it.  (I was able to access it and use it 2 years ago.) I've done all the usual - made sure PKSolver was active, and the location of PKSolver.xla was "trusted".  

Windows 10, MSOffice is 2016, I'm using Office 365