When I tried to create a user Max Jackson, In the roles there is no license option of Salesforce , So I created the user under Salesforce option. While trying to add Max Jackson in Sales Processing assignment group. I got an error saying u cannot add this person. Then I started debugging the issue to know where it went wrong. Then I realized I assigned the wrong license. After digging much deeper I found out that we can assign only 2 users to salesforce license. Then I found which user is not in need for me and then I changed that user to another License and assigned Max Jackson to the salesforce license. Now I'm successfully able to add Max Jackson to the Sales Processing permission group.