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The usage of Linked issues depends upon the permissions. So, there are two things:
1) Permission to linked issue
2) Permission in Jira Project
The users won't be able to linked issues until it allows them, it could be either of them. Can you please tell me which instance of JIRA you are using? Is it Cloud, Server or Data Center?
Note: Upvote, Like or Accept Answer if it resolves your query.
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