Hello @Haroon Habibi .
You should probably check to see if you're in a group that has Browse Users and Groups global permission.
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So, for this I refer to a previous Community post: https://community.atlassian.com/t5/Jira-Service-Management/Unable-to-tag-people-in-Jira-comments/qaq-p/2107503
Which pointed to this Knowledgeable article: https://confluence.atlassian.com/jirakb/mentions-and-autocomplete-are-not-working-in-jira-cloud-779160727.html
Note: You have to be a Jira Administrator (Jira Project Administrator is not enough) to be able to do this.
Good luck!
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