Ensure there is no old DLL file named GleamTech.Core.dll anywhere in your project, it was replaced with new DLL file named GleamTech.Common.dll since Version 7.3.0 - November 20, 2019
In Visual Studio: Go to Tools -> NuGet Package Manager -> Package Manager Console and run these commands one by one in order:
Uninstall-Package FileUltimate -RemoveDependencies
Uninstall-Package FileUltimate.NuGetOrg -RemoveDependencies
If you are using direct DLL references instead of NuGet reference, first clean your project, remove references to old assembly GleamTech.Core.dll, add references to new assembly and rebuild your project. Ensure there is no GleamTech.Core.dll left in your project bin folders.