First of all Please make sure you have created Plans and now you want to assign any plan to any user?
1- Go to WordPress Dashboard --> Pricing Plans--> Here you will see your All Plans which you have created. Now Click on Edit for that plan which you want to assign to any user.
2- In Meta boxes you will see option "Put here USERNAME to assign this plan for particular user" --> Put a user name here like "demo" without comma please.
3- Update Plan.
That's All.