On page 6 of the release notes:
A K chassis supports a maximum of 12 × 10Gb ports. In addition, slot 10 in a K10 chassis only supports 10Gb line cards. Unsupported configurations cause an error message to appear to the console and the problem line card does not power up.
Release note link: https://documentation.extremenetworks.com/release_notes/EOS/9035486-01_K-Series_RelNote_FW0863020004.pdf