Windows R Installer. V 5.0.22621.1 error 1639

Trying to install update of Dell Support Assistant and get Windows Insaller v5.0.22621.1 pop up. Click on OK then get error code 1639 which I understand is ERROR_INVALID_COMMAND_LINE.

Tried following:

At cmd prompt > run as administrator ran sfc /scannow. No errors found.

At cmd prompt > Dism /Online /Cleanup-Image /CheckHealth. No issues found.

At Run, ran msiexec /unreg then at Run, ran msiexec /regserver. Still no joy at running updates.

Also tried the following commands at Run:

Commands:

• %windir%\system32\msiexec.exe /unregister

• %windir%\system32\msiexec.exe /regserver

• %windir%\syswow64\msiexec.exe /unregister

• %windir%\syswow64\msiexec.exe /regserver

No joy.

Went to Start > services.msc > Windows Installer > Startup type tried to click the drop down to change to Automatic BUT could not change from manual as in lowkey

Any ideas welcome. Thanks in advance