Can i enable one specific app but keep others disabled?

Yes. Do Not select a category for App Limit, select only the apps that you want to disallow.

Read: Select all other apps except Discord

KermitGobush Said:

"Can i enable one specific app but keep others disabled?: My child uses discord to talk to his friends but with child restrictions you cant. Is there any way to enable it while keeping other things disabled?"

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Singling Out Apps:

Disable these apps in Screen Time: Use Screen Time On your iPhone, iPad, or iPod touch - Apple Support

Please see the pic below which is self-explanatory (In your case unselect only discord app in the category and Select all other Categories