Linguists cannot lock/unlock segments. Only PM/Admins are allowed to.
Available workaround (if any):
If enabled in project settings, Linguist can edit the locked segment by clicking on F2 button. However, there is no workaround for other cases apart from contacting respective PM or Admin user.
Linguist can lock/unlock segments if enabled in User settings.
For example, locked and unlocked segments need to be joined. If enabled for experienced Linguists, it would not have to be forwarded to PM or Admin and no extra time would be needed.