Can't dig in minecraft

This sounds like you are in Adventure mode. If you've done /gamemode survival and it still doesn't work, please provide more details as to what tool you're using and what you're trying to break.

If that command doesn't work because you don't have permission, you can enable cheats by opening your world as a LAN world and enabling cheats, and that should allow you to run the command.

If you're on a server that you don't have permission on but you should be able to break stuff in, contact whoever is running the server or a server admin.

If you're on an Adventure / minigame server then... there's nothing wrong and your mining ability is disabled by design.