を区別できない

cannot distinguish

→ を
phrase Rare #32,636
cannot distinguish Used when someone is unable to tell two or more things apart. The を marks the things being distinguished.
JA: 私は双子の二人を区別できない。
EN: I can't tell the twins apart.

動物は虚偽真実を区別できない

Animals cannot distinguish falsehoods from truth.

彼は本物偽物を区別できない

He can't tell the real thing from a fake.

現実を区別できないのか

Can't you tell the difference between reality and dreams?

— オープン・ユア・アイズ (1997)

それとこれを区別できない

I can't tell this apart from that.