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.

Depending on how recent your install is, it may just be quicker to revert to a prior system restore point, so long as you have the feature enabled. They're pretty fast and pretty failsafe from my experience.

From the Recovery options in Windows 10 support article:

This option takes your PC back to an earlier point in time, called a system restore point. Restore points are generated when you install a new app or driver, and when you create a restore point manually. Restoring won’t affect your personal files, but it will remove apps, drivers, and updates installed after the restore point was made.

1. In the search box on the taskbar, type control panel, and then choose it from the list of results
2. In the Control Panel search box, type recovery.
3. Select Recovery > Open System Restore.
4. In the Restore system files and setting box, select Next.
5. Select the restore point that you want to use in the list of results, and then select Scan for affected programs.
6. You'll see a list of items that will be deleted if you remove this restore point. If you're OK with the deletions, select Close > Next > Finish.

Notes

• If you don’t see the restore point that you want to use, select the Show more restore points check box to see more restore points.
• If you’re not seeing any restore points, it might be because system protection isn’t turned on. Here’s how to check:
  1. In the search box on the taskbar, type control panel, and then choose it from the list of results.
  2. In the Control Panel search box, type recovery.
  3. Select Recovery > Configure System Restore > Configure and see if the Turn on system protection option is selected.
     • If the Turn on system protection option is not selected, system protection isn’t turned on and there aren't any restore points. In this scenario, you won't be able to recovery your PC using a system restore point and will need to use one of the other recovery options listed on this page.