Hi @Richard ,
Did you try to install the latest version of SSMS 18.12.1? You can get this from this MS link.
If it is not work, please try the solution from this similar thread.
>If you encounter this error with the reason being "ssms is currently running", check the file name of the setup exe. If it's ssms.exe, rename it to anything else, I suspect the setup is detecting itself as being management studio based on file name. This worked for me.
If It is still not work, please check your SSMS setup log(such as %TEMP%\SSMSSetup\SSMS-Setup-ENU_20190430231104.log) to find more related error message and share them to us.
If you cannot find more information from SSMS setup log, please use process monitor to capture the SSMS installation process, then find the useful error information from this.
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