Signing commit with OpenPGP subkey fails

Git uses gpg by default, which is GnuPG 1 on most systems and does not support elliptic curve cryptography. As your primary key is an elliptic curve key, GnuPG 1 cannot use the key at all. You will be able to observe the same when trying to use the key with GnuPG (gpg --default-key key-id --sign).

Configure Git to use gpg2 instead, which is required to be at least GnuPG 2.1 (which you have, as you can use the elliptic curves key):

git config --global gpg.program gpg2