You must be a Jira administrator to give Jira users permission to comment on service desk issues.
To give other Jira users permission:
From your service desk project, go to Project settings () > Permissions.
Select Actions > Edit permissions.
Under Comment permissions, select Edit for the Add comments permission.
Under Granted to, choose Application access.
From the dropdown, select Any logged in user.
Select Grant.
Other Jira users can make internal comments, but cannot share comments with your customers.