Fields description should be edited while creating and then during closure only.
See https://community.atlassian.com/t5/JIRA-questions/Every-one-should-be-able-to-create-ticket-But-can-be-edited-when/qaq-p/598900?posted=598937#U598937
You are asking some very basic questions here that strongly suggest that you have not used JIRA or read any of the documentation, and you are repeating them.
I'd recommend that you contact an Atlassian Solution Partner and get some basic training in, or contact your administrators and ask for their help.
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