Resolution field of tickets have been set as a mandatory field. Can I keep this field empty (as"None")?
In your field configuration you can edit the field properties to be required/optional. It could also be that you have a postfunction set in your workflow which fills in the resolution field for you. If that is the case, you can delete this post-function on your transition to 'done' in the workflow.
This is the same as what you asked the other day over at https://community.atlassian.com/t5/Jira-questions/Problem-with-resolution-and-status/qaq-p/813822
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