You have to ask this question to Letterboxd, not to us.

@Juju' said:
Tv shows
Hello!
I’m sending this here without knowing the site, but I have a question I’d like to know the answer to.
How can a series be added to Letterboxd if it is already added to the data site ?
For example I wanted to be able to add Ash vs Evil Dead (And other series) but the series already exists on the site but is not available in the application...

Letterboxd, like any other service, uses data from TMDb.
But they are independent.
They are not partners or belong to the same group or have the same administration.

In your case, if the movie or TV Show already exists in the TMDb database
https://www.themoviedb.org/tv/62264-ash-vs-evil-dead
then you should try to solve your problem directly with Letterboxd.

Okay! Thank you for your answers.