Sometimes after upgrading Jira instance, Secure Fieilds plugin shows an app might be shown as incompatible and the regular update procedure won’t work. Different It is caused by differences in JIRA Java API, so different Jira versions may require a different package of Secure Fields because of differences in JIRA Java API.
In order to fix it, you will have to uninstall the app manually and install the current latest version by searching for Secure Fields in Find new apps section. All settings and license will remain unchanged.