Classic custom fields are now outdated. You can update them to the new custom fields through this screen.
Choose the classic custom field you want to upgrade, and then click the Upgrade button.
Your existing custom fields will be transferred to the new custom fields system.
Learn more: Custom Fields
Please ensure that the upgrade has not caused any issues. If you encounter any problems, you can reverse the operation by selecting Undo in the History section.
Learn more: History