This is an ellipsis. You can include the implied words in your translation. Paul asks this question to get the attention of his readers. AT: "Why could they not attain righteousness?" (See: [[rc://en/ta/man/translate/figs-ellipsis]] and [[rc://en/ta/man/translate/figs-rquestion]])
This refers to things that people do to try to please God. You can make this explicit in your translation. AT: "by trying to do things that would please God" or "by keeping the Law" (See: [[rc://en/ta/man/translate/figs-explicit]])
You can indicate that Isaiah wrote this. You can also translate it in an active form. AT: "as Isaiah the prophet wrote" (See: [[rc://en/ta/man/translate/figs-explicit]] and [[rc://en/ta/man/translate/figs-activepassive]])
Both of these phrases mean basically the same thing and are metaphors that refer to Jesus and his death on the cross. It was as if the people stumbled over a stone because they were disgusted when they considered Jesus' death on the cross. (See: [[rc://en/ta/man/translate/figs-doublet]] and [[rc://en/ta/man/translate/figs-metaphor]])