This is a general problem for words starting with an apostrophe -- there are hints stored for them but they are not displayed.
Duolingo's technical staff know about this and are working on it, so there's no need to mark "hints wrong or missing" for this case: it's a general, known bug already.
'IH means "be beautiful, pretty".
Klingon doesn't have adjectives as a separate class of words, distinct from verbs.
Instead, there are some verbs that describe certain qualities -- for example, 'IH could have the dictionary translation "be beautiful".
In this sentence, it's a full verb -- qach is the subject and it is after the verb 'IH.
So 'IH qach means "The building is beautiful." 'IH here means "is beautiful" here (or "was beautiful" or "will be beautiful"), and qach means "the building" or "a building". It cannot be translated just "building" in this sentence because "Building is beautiful" is not correct in English, if "building" has the meaning "structure, edifice" -- countable nouns in the singular need an article or other determiner in front of them.
Your phrase "(a) beautiful building" would be qach 'IH -- when such a quality verb comes after the noun, it acts like an English adjective.
(Normally, a noun followed by a verb would be the object of the verb: for example, paq laD would be "he/she reads the book". But quality verbs don't take objects, so qach 'IH is unambiguous; it cannot mean "it beautifuls the building".)
A qach can be any building -- it could be a house but it could also be a library, hall, museum, hospital, ....
So "house" is too specific a translation. Kind of like translating "vehicle" with "car", or "animal" with "horse".
A house is a juH qach: a building that's a home.
Because the adjective-verb stands before the noun, not after it.
Remember that Klingon is object–verb–subject -- when you have 'IH qach, the order is verb 'IH then subject qach, so 'IH is translated as a verb, "is beautiful".
If you want to say "a beautiful building", the adjective-verb has to stand after the noun: qach 'IH.
(That can't be mistaken for object + verb, because Klingon adjective-verbs can't take an object; qach 'IH cannot mean "it beautifuls the building" and can only mean "a beautiful building" or "the beautiful building".)
And there is no "it is" in the sentence -- that would be 'oH in this case, for qach 'IH 'oH "it is a beautiful building".