"This is our restaurant!"
Translation:Das ist unser Restaurant!
If "this" is standing in front of a noun, it is translated by a matching (declined) form of "dies", e.g. "this restaurant".
But if "this", "that" and even "these" and "those" stand alone as in "this/that is" or "these/those are" you say "das ist" resp. "das sind". You can use "dies" instead of "das" for "this", but not a declined form: "das ist unser Restaurant" or "Dies ist unser Restaurant", not "dieses".
It would get strong inflection if you were using 'ours' to replace a previously-mentioned noun. For example: "Is that restaurant yours?" "Yes it is ours" (Ja, es ist unseres). Here, it doesn't get strong inflection because it comes before the noun.
Why the setence: "Dieses ist unser Restaurant!" is incorrect? Duolingo says that "dies" is the correct answer instead of "dieses". But "dies" is nowhere in the Grammar section. How do I suppose to know that? I'm learning German, I don't know the language. Duolingo is obligated to show me everything about grammar, and not letting me guess what is the correct answer!
"this is" can be translated as "dies ist" (less common) or "das ist". "this" etc. standing alone are treated differently from "this" etc. used attributively (i.e. in front of a noun phrase).
"unser" is (like all possessive pronouns) inflected, so it has different forms depending on case, number and gender of the qualified noun.
See the full tables here: https://study.com/academy/lesson/german-possessive-pronouns.html