This is probably because enough Gold seats have not been allocated to the group or subgroup.
In the following situation, only 2 Gold seats from 5 available ones have been used, but it is not possible to assign Gold seats to more kids. Why?
This is because the kids are in a subgroup with only 2 Gold seats. First, you must edit the number of Gold seats available in that subgroup or move kids who still have available spaces.
How to update Gold Seats in a group or subgroup
From your Enterprise account homepage, click on Go to Groups and select the group or subgroup where you want to update your Gold Seats:
Click on Info from the group or subgroup menu and update the number of Gold seats allocated. Don't forget to click Update to apply the changes.
Check out this article to learn how to assign or remove Gold to kids inside your groups/subgroups: How can I give/remove Gold seats?