It looks like it's a silly sort of song with lots of funny phrases. "He can't be bleat" is not a proper sentence. The proper sentence would be "he can't be BEAT", which means something like "he can't be defeated and you're going to die". I think it's just playing with words. "Bleat" is the sound a lamb or a goat makes. "I heard a sheep bleat in the field". Lambs and goats are cloven hoofed animals, so in English mythology they are associated with devils and demons (who are also cloven hoofed). So the Netflix show is about a "witcher" and he or she probably hunts devils and demons and witches, etc.. But the show must have a silly sort of feel to it because the song uses a lot of silly little rhymes like this.

Андрей, я примерно так же подумала, но меня смущает, что в некоторых случаях в тексте песни это "he can't be bleat" берут в кавычки, типа "И ведьмак закричал: "he can't be bleat". Загадочно)

Oh, I see. Yeah, it makes sense, it's not meant to be a very serious song. Thanks Andrea!

Ведьмак так ревел, он не может скулить или блеять. Имеется ввиду, Ревел громко, а не был скулящим.

Hi, American here, so maybe other English speakers around the world will have a different opinion, but that phrase is nonsense. It is not an expression I have ever heard. It seems like a play on "He can't be beat" which means he will always survive, that he is superior. The video and song seem like a parody, so perhaps the artist just made up an expression that fits the style he is trying to create.