"Permission Scheme" set like the follow picture.
but "Assign to me" icon is not shown in issues.
Are you the reporter or in at least one of those three roles? That permission gives you the right to assign the issue. It gives you "assign to <someone>" rights.
But it does not mean you are on the list of possible assignees. Take a look at the other assignee permission, which defines "who can be assigned an issue" - I suspect you do not meet the rules for that. So you can assign issues, but not to yourself because you're not on the list.
The note says "Ability to assign issues to other people." I feel that I should be able to assign myself.
As @Nic Brough -Adaptavist- said, you need to be on the assignable list. It's how the product works.
I am on "Assignable User"list, but not on "Assign Issues"list.
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