Trisquel 8: Lightning for Icedove

Dear Trisquel users,

Please excuse me for my ignorance.

Facing the same challenge as [1] (thread locked), I did as advised but still do not see Lightning in the add-ons manager of Icedove.

Thanks in advance.

[1] https://trisquel.info/en/forum/trisquel-8-lightning-icedove