Renata: Only a limited selection of films and TV shows are available for watching on IMDb. Those that are available can be found in the Freedive section (see https://www.imdb.com/freedive/). If you are in the United States and logged in to IMDb, you can watch any of the titles available in Freedive, but only those titles. (Freedive is not available outside the U.S. currently.) There are currently 331 feature films available through Freedive, plus some TV series, documentaries, and other programming.
The "watchlist" on IMDb has nothing to do with being able to actually watch a film. It is just for your own reference.