Please remove the Smart Checklist Free Trial add-on from the "Manage add-ons" Admin section and install Smart Checklist Pro from the Marketplace.
All your data i.e. created checklists or templates are safe and will not be removed. They are stored on your Jira instance.
Detailed process
Go to "Manage add-ons"
Find "Smart Checklist for Jira. Free"
Click "Uninstall"
Go to "Find new add-ons"
Type "Smart Checklist for Jira. Pro"
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