Hello Everyone,
I just wanted to pass along an issue that I had for a very long time and no matter how many forums and searches conducted on the subject no suggestions worked for me. However one person mentioned .Net Framework but suggested uninstalling, which I ignored.

I could not load the Solver Excel Add-in using the traditional methods via Excel in both 2016 and 2019. I would always get a message that the file could not be found or Excel would just crash. I even upgraded Windows 7 to Windows 10 hoping it would fix the issue. It did not.

Here is something I tried and it worked.
I just recently installed Office 2019 from Office 2016 and based on the one mention of the .Net framework I decided to open the Solver.XLAM (copy) using Visual Studio 2019. However, I did not get Visual Studio 2019 outright, I got Microsoft VS Version Selector, so I picked that. When a dialog box came up I had a few choices, I picked the one to Trust This Publisher, which is Microsoft, and low and behold Solver loaded to my Data Tab. This was in addition to already having Microsoft as a Trust Publisher in Excel. Now when I open Excel the Solver loads without issue.

So if you hear of someone with this issue, please pass this along. I can only assume if the are using Office 2016 they could try opening an earlier version of Visual Studio. I can only assume Community would do the same. I am an instructor so I have a full blown copy of 2019.