If I am not wrong, the user is not able to Assign the issues under this project to any other user. You need to grant this user, the permission called Assign Issues under the INFRASTRUCTURE project permission scheme.
Browse project INFRASTRUCUTRE.
The user will now be able to see Assign button for the issues under the INFRASTRUCUTRE project.
Refer this link to check the different permissions for a project.
Hope this helps you!
This is probably not the best way to do it - what you really should do is use roles (or groups, or other methods) to grant the permission to the user. The principle is correct - your user is missing a permission, but adding a user to a permission scheme is blunt, unwieldy and unfriendly. For a start, if you do it this way, if the scheme is used by any other project, then they'll get assign rights in there and that could be utterly wrong.
Your best bet is to compare their groups and roles with another user who can use the assign option. I expect you'll find differences in roles.
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