How many boards do you currently have in total in your Free Plan team? The user with the role of Owner will always have edit permissions, so it sounds like you have at least four baords and one of your older boards was set to view-only status. If this is the case, one or more boards will need to be deleted Ilin order for your board to become editable again.
If this is the only Miro team that you are member of, you could also leave this team, at which point Miro will prompt you to create your own Free Plan team. Once you have done this, you could have your teammate add you back to the old team - now your Miro account profile is a member of two teams. Then, you could duplicate your old boards and move the new duplicates to the Free Plan that just created.
Here's a post where I detailed these steps. In these steps, you can even duplicate the board's to a new Free Plan team without first being added back to the team. Note: this may not work for boards that are in a vew-only status as I am not sure if you can modify a view-only board's Share settings.
https://community.miro.com/ask-the-community-45/free-version-specific-limitation-in-terms-of-users-6096#post25358