I'm not that familiar with Zephyr, I've not seen that error except for when indexing (Zephyr and JIRA) was needed, although I saw more of an error message.
Try reindexing both, and try again. If that fails, the next step is to start reading the logs.
For unlocking the fields you can refer to the Jira's KB https://confluence.atlassian.com/jirakb/how-to-unlock-a-locked-field-779158866.html, if it is a blocker for you.
Zephyr will allow traceability for Epic as well, please ensure atleast a test is linked to that Epic. Here the link is the Jira's issue link. If you still face the issue we are happy to assist you on this issue through our support ticketing system https://support.getzephyr.com/hc/en-us
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