Seems counter intuitive but they probably need to verify identity. Should be no problem opening in branch. Unless you're faking your identity, then maybe it'll be a problem.
Many reports of Chase not allowing online opening but allowed opening in branch. If it was something else causing the rejection I don't think they'd offer the in person option. Our first Chase accounts had to be opened in person because they didn't have branches in the state so the system couldn't assign some number. Fortunately we live close to the state line and it was easy. Could be something logistical like that as another option.
You may be able to see the letter in your online account so you don't need to wait for snail mail. Could try looking in Notices and letters a day or two after application to see if it's there. May give you a better idea of why, or just repeat the offer to open in branch.
If it's not the $900 offer, there's still a few more days to try for that offer. Public link is still open -
https://www.doctorofcredit.com/targeted-chase-900-checking-savings-bonus/