This may or may not be related to a bug where reserved field names show the same error: https://jira.atlassian.com/browse/JSWCLOUD-18497
We are working on a fix. In the meantime, can you please take a look at your fields and see if any custom field has name that is used by Jira internally? Renaming should do the trick and allow you to create new issue types. (It is not necessary to delete fields. Rename is enough.)
Example: field Resolution will show error. Resolution2 will be fine.
I understand it is difficult to find out which fields are used internally; if you post your field names I can check if there is a conflict.
Please let me know if that worked.
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