I suspect you have the "Field security" add-on and it's been configured to block your access to the field. You'll need to get the config changed.
Ok, the padlock is not a native JIRA function generally. If you can't edit a field, JIRA will tend to hide it or grey it out, not put padlocks next to it.
What type of field is it?
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That doesn't tell me anything about the field, or what the problem is, it's a permission.
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