Solution cannot be loaded because .NET Framework 3.5 is not installed

Error

“The following solution cannot be loaded because the .NET Framework 5.5 is not installed Merlin.vsto” when opening Excel after installation.

Solution

Locate the folder:

C:\Program Files(x86)\Common Files\Microsoft shared\VSTO\10.0

Where 10 is the Excel version number so may be different on your pc

Rename the file VSTOInstaller.exe.Config   to VSTOInstaller.exe.Config.old

Links

See https://support.microsoft.com/en-gb/help/3176961/cannot-install-vsto-add-in-targeting-net-framework-3-5-to-office-2013