"Your watchlist is full. Remove some of the TV shows or movies and try again"
Just tried to add "THE STAIRCASE" to my watchlist, but system refuses, says I need to delete some already there.
Is this a new restriction, as only have 10 shows listed?
A figure of 99 Watchlist folders can be added to your account if I remember correctly (might be wrong though).
If you can only add 10 programmes to your Watchlist then it sounds like a glitch or bug.
Have you tried on more than one device to add more TV shows or movies to your list?
I haven't switched on my NOW playback devices yet to see if the problem is more wide spread.
Also try adding a different TV show has a temporary test just incase there is a glitch / bug with "The Staircase" programme.
I just added The Staircase to my Watchlist with no problems for me on my phone where currently I have 14 Watchlist folders saved.
Perhaps get in touch with NOW live chat to report it happening on your account if other devices doesn't work.
Fortunately my NOWTV app on my TV allowed me to add😉, unlike on my laptop.