Unable to uncheck "Start" workspace in InDesign CC 2019

You don't need to reset your preferences.

You know when you have document open, there's the workspace switcher in the top right hand corner? Just change your workspace to something other than 'start'. It should then bring back the greyed out option in preferences. 

Worked for me anyway.

Are you running InDesign CC 2019 in trial mode?

If yes, I fear you can do nothing about this.

Regards,
Uwe

I tried both methods, but none of them worked. Maybe I should accept the "start" screen.

If resetting preferences doesn't work try deleting your InDesign cache files. This can be helpful for problems within the interface. To do so:

For Macintosh Users: The User Library folder in which InDesign’s cache files are stored is hidden by default on most Macintoshes. To access it make sure that InDesign is closed and click on the desktop to launch a Finder Window (Command-N).With this window in column view follow the path User>Home folder (it’s the folder with an icon that looks like a house—it may have the user’s name rather than “Home”) and click on the Home folder. With the Option Key pressed choose Library from the Finder Go Menu. “Library” will now appear within the Home folder. Within the Library folder find the “Caches” folder. Within the Caches folder find and delete the entire folder “Adobe InDesign”. I find that deleting the InDesign cache folder completely leads to a lasting change. 

For Windows Users: On Windows 7 and above the caches files are hidden. To find them go to the Control Panel and open Folder Options and then click the View tab. Then select “Show hidden files and folders” or “Show hidden files, folders or drive options” in Advanced Settings. Then delete (or rename) the folder at the end of this path: C:\Users\[User Name]\AppData\Local\Adobe\InDesign\Version [#]\<Language>\Cache.

Restore your preferences as described here:

Set preferences in Adobe InDesign