As it is my first post, hi everyone
I found that OSMC installer repository key is expired for some time already. Also, there is no repo for Ubuntu 18.04 or Debian 9 (but 16.04 installed on 18.04 works perfectly fine).
$ apt-key list "A9AA 7116 51DB B111 4959 5B95 8873 E7BF 70F1 BBEA"
pub rsa2048 2014-10-14 [SC] [expired: 2019-03-09]
A9AA 7116 51DB B111 4959 5B95 8873 E7BF 70F1 BBEA
uid [ expired] home:osmc OBS Project <home:osmc@build.opensuse.org>