While setting up Yamete on the parent's iPhone, the Screen Time approval fails if Content & Privacy Restrictions is switched on — switch it off before the setup reaches the Screen Time step. Yamete cannot switch it off for you: Apple gives no app the ability to change this setting.
On the parent's iPhone
Open Settings → Screen Time → Content & Privacy Restrictions and switch it off. If a Screen Time passcode is set, iOS asks for it.
If someone else manages this iPhone's Screen Time
When the phone's Screen Time is managed through Family Sharing, the toggle is on the organizer's device. On their iPhone open Settings → Screen Time, tap this family member's name under Family, then Content & Privacy Restrictions → off. The organizer's Screen Time passcode is needed.
A work or school iPhone
A managed device can keep the setting on through its management profile — the administrator has to change it.
Then return to Yamete and approve the Screen Time prompt. Setting up a child's iPhone next? The same check applies there — if the child's Screen Time is managed through Family Sharing, the switch lives on your phone under the child's name.