IMEX failed to complete: rPGP error: no matching packet found

in general, Delta Chat is strict in which keys are supported. eg. OpenPGP packets must match Autocrypt Level 1: Enabling encryption, avoiding annoyances — Autocrypt 1.1.0 documentation, cypher should be Ed25519 plus Cv25519, or RSA

iirc (i am not deep in that topic), this format is used by many todays PGP implementations.

supporting arbitrary, maybe legacy, secret keys is not a primary goal and is always weighted carefully - eg. a larger codebase also means more security risk here. not to speak about implementation and maintenance effort.

some links to - maybe helpful - similar topics: