I have a total of 6 playlists, but when I go to add additional videos, the only option is to create a new playlist, not add to an existing one. Does anyone know why that could be?
I also can’t add a video (movie) to an existing playlist. Is this normal?
If this playlist has a small ‘cog wheel’ icon on top of its poster, it is a ‘smart’ playlist.

You cannot add items manually to a smart playlist. They are updated solely according to the filter which was used to create them.
You can create a ‘dumb’ copy of a smart playlist. This then allows you to add items manually to it.
I do have that cog wheel icon on a few playlists, but when creating a playlist from videos created from the “Other Videos” template, it automatically creates a smart playlist.
I saw your link about converting it to a dumb playlist, but I’m not seeing the “Up Next” part after clicking on the playlist. I can provide some screenshots if needed.
Creating a new playlist isn’t a big deal, but I don’t want it to default to a smart playlist. I’m perfectly happy with a dumb one 
@DJQuad said:
I do have that cog wheel icon on a few playlists, but when creating a playlist from videos created from the “Other Videos” template, it automatically creates a smart playlist.
You must ‘select’ items. Otherwise you get a smart playlist.
I saw your link about converting it to a dumb playlist, but I’m not seeing the “Up Next” part after clicking on the playlist.
Don’t click on the playlist. Play it.
Creating a new playlist isn’t a big deal, but I don’t want it to default to a smart playlist. I’m perfectly happy with a dumb one
https://forums.plex.tv/discussion/219448/howto-create-playlists-updated-for-new-ui