This is what it says on the tin.
(Edit - sorry, I should have said this originally, but got stuck into the answer instead and forgot).
But it's not a well-labelled tin. The error is short, as they should be, but doesn't really tell you what it actually is, let alone why it's that way or how to fix it!
When you raise a request, there is a Jira issue that needs to be created behind it (A request is a layer in front of an issue for the more simple and public thing the customer sees.)
In this case, you have configured the issue such that it has a default assignee that can't be assigned issues in that project. The issue needs to have an assignee but the one selected (or the absence of one) means Jira can't set it.
Check the project configuration for the default assignee, and check the global setting for "unassigned issues are not allowed"
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