I would suggest you use the action “Office 365 Outlook – Send an approval email”.
When testing this issue on my side, with the action “Send an approval email”, it works fine with a group email address. While with the action “Start an approval”, it returns the error message you mentioned when a group email address is used.
I will help confirm this issue on my side and back to you later.
Best regards, Mabel Mao
Community Support Team _ Mabel Mao If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.