Hi Ignacio, I had the same problem as you did. I searched for a solution, and for me it was that I had to open "Outlook (classic)", which works with MS Word mail merges, but "New Outlook" does not. If you search "outlook" in your programs, it should pop up as a suggestion. Why this would be the case, I don't know.
If you try this, be aware that pending mail merges will also process once you open Outlook (classic).
I hope this works, and here's hoping we can both move away from Microsoft products soon. Cheers!