OneDev issue fields are fully customizable. You may set the Assignee field to allow empty in administration / issue settings / custom fieds

I tried adding a "date" type, which allows me to select it in the Due Date Issue Field when import jira. However, after adding a user field, it still doesn't appear in the Assignee dropdown. Actually, the system already has three default fields for issues: Assignee, Type, and Priority. The default Assignee field is already set to "allow empty," and I've also tried adding a user field, but it still doesn't work.

My mistake. It expects an issue field of user type with "allow multiple" set to true, since jira issue can be assigned to multiple users. By default, OneDev assignee field is single valued, and you may change it in issue field setting.

Also please upgrade to 11.6.10 as it matches users in JIRA and OneDev by email address instead of full name which is more practical.

Thank you so much for your prompt and timely support. After enabling "allow multiple," the user field appeared. I'll go ahead and upgrade and continue with the import.