SQL Server Management Studio SSMS 18.0 General Availability release (GA) install fails
From the command line, I was able to uninstall a different 18.0 preview (I don't have 6 anywhere):

msiexec /x "{2051D5D4-618D-4B9F-BA64-C1AE19003F2E}"


But I did not have the error about the shell msi, and I'm not sure if the CLSID changed, so I'm not sure that will work for you. (I'm not convinced the GUID in the dialog you're getting is the one you need for SSMS.)

Stolen shamelessly from my own blog post about uninstalling Evaluation Edition which fails if the product has expired.