SuStel:

The head noun of a relative clause must be the subject or object of the clause

Ok, I think I understand this.

SuStel;

and the head noun must be the noun that fits into the main sentence.

I'm afraid I don't understand this. Can you explain this further? Or
perhaps just write an example?

The head noun of the relative clause is the "anchor" that fits into the main sentence, with the rest of the relative clause dangling from it.

wov wa'Hu' HuDvo' Hov leghpu'bogh HoD
The star that the captain saw from the mountain yesterday was bright.

The relative clause is wa'Hu' HuDvo' Hov leghpu'bogh HoD the star which the captain saw from the mountain yesterday. The head noun of the relative clause is Hov. If you were to include this noun in the main sentence without the rest of the relative clause, the sentence would still work: wov Hov The star was bright.

The head noun of a relative clause needs to fit into the main sentence in this way, and the head noun of a relative clause must be the subject or object of the relative clause. Therefore, it is not possible to construct a relative clause like DujDaq jIHaw'pu'bogh ship in which I fled because the head noun is not subject or object of the main clause, and we know that Duj is your intended head noun because it is the noun that fits into the main sentence: Duj vIleghpu' I saw the ship.

That's why the ship in which I fled is a no-no in Klingon.

-- 
SuStel
http://trimboli.name