The Free trial is limited to 30 days or 20 items available for creation. After that the board will turned into a read-only mode and you'll be asked to upgrade to one of the paid plans.
You can find details and subscribe on the pricing page or upgrade directly from the app.
FAQ
Q: Can free version users access and interact with shared checklists?
A: Yes, users can access and interact with the checklist within the limits of the free tria version.
Q: Do users need a paid version to make changes to shared content?
A: Users are able to make changes to the shared content within the limit of the free trial version.