Hi Ximo,

For the Free Plan, only the 3 most recently created boards in your account are available for editing and collaboration with your team. The rest of your boards are inactive and are locked in a view-only mode. To edit inactive boards you can delete unnecessary active boards or upgrade your account.

As a workaround, if you'd like to work on a particular board, you can delete unnecessary active boards, upgrade your account or duplicate the necessary board and the copy will become active. Please note that this will push the oldest active board to inactive mode.

I hope this helps!