How do I remove this? If possible- and why does it say this on this persons account

They're either banned, you blocked them, or they blocked you

Look at your settings TT might have automatically filtered certain topics on your phone. If it hasn't then maybe the video creator has filtered theirs so not everyone can watch.

I've noticed problems watching occasionally and each time I've discovered the settings have changed especially if I've reinstalled the app

You're blocked sorry