The original sentence is correct. Un bloc, două blocuri is a neuter noun, and since the singular form is used, it is the same as being masculine. So we pick the matching singular masculine form of the adjective in this case, which is 'vechi'. 'Veche' is the feminine form, like for example 'casă veche' - old house.
I suppose the ending -i is throwing you off, 'vechi' has the same form for both masculine singular and plural, as well as feminine plural.
It's been a year since the last comment and "block of flats"is still marked as wrong. I believe Duolingo is supposed to help me learn Romanian, not American English. "Block of flats" is NOT wrong. Please fix this.