Mourning
noun, の adjective
The kanji and the word are exactly the same. That means they share meanings as well.
も
Since this word is made up of a single kanji, it probably uses the kun'yomi reading. You did not learn this reading, so here's a mnemonic to help you: What's the first two letters of the word mourning? It's mo (も). You can use that to remember.
大変申し訳ありませんが、喪に服しているため年賀状が送れないのです。
I’m very sorry but I’m afraid I can’t send a New Year’s card because I’m in mourning.