[2021] Purchased DLC unavailable?

If you're on PS5, try:

• Open PS5.
• Head to your Games Library.
• Select the game you are having trouble with and press the options button.
• Select “Manage Game Content”.
• Now, you will be able to see all the data that you have installed of that particular game to date.
• After Scrolling Down, you will see some files that you have downloaded but have not installed.
• So here, all you have to do is click on the install button, present in front of the files
• Now, if you have installed all the files, but they are still not showing up in the game, delete them and install them again. This shouldn't be a tiny install since it's just a licence -- the content itself is in the main installation.

I believe there is a similar process on Xbox Series S|X but I'm not particularly familiar with the interface.

Edit: Google suggests

• If your game is running, shut it down.
• Press the Menu button on your controller.
• Choose Manage games and add-ons.
• Select the game at the upper right of the screen.
• Choose Manage installation on [drive name].
• This brings up a list of items including the game itself and any available add-ons.
• Uncheck the checkbox beside everything except the game itself.
• Select Save changes and wait for the uninstallation to finish.
• Reboot, and reinstall the addons with the same process above, only re-checking the checkboxes beside the DLC.