Error On Linux Mint Cinnamon 19

[user-removed]

Plan

Free/Premium

Country

Turkey 

Device

(MS-1632 AMD Turion(tm) 64 x2 Mobile Technology TL-64 * 2)

Operating System

(Linux Mint Cinnamon 19)

My Question or Issue

 I have a error when i want to update my applications . It says :

Err:10 http://repository.spotify.com testing InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A87FF9DF48BF1C90
Reading package lists... Done 
W: GPG error: http://repository.spotify.com testing InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A87FF9DF48BF1C90
E: The repository 'http://repository.spotify.com testing InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

I can't install on snap because Its not supports amd64 procressor . Help please