Can’t fill this hole pls help!

[shaprlon]

Hey I’m having trouble filling/deleting this hole. It works on others but on this one one side of the body is curved maybe that’s why it’s having problems. Anyways I need to close this so I can use the whole shape to cut it out of another. Pls let me know if someone knows how to solve this. Thanks!

Ps. Unfortunately I can’t upload videos yet as I’m a new user

[Oregonerd]

Create a peg so to speak to plug it using the Boolean add tool. Use the replace face tool to match the surfaces if needed on the curved surface. Been there done that.