In simple terms - これ is a demonstrative pronoun and means "this". It replaces a noun. Let's say you're holding an apple and say 「これが美味しい」You're replacing the word りんご with これ. You can't say こんなが美味しい.
こんな is used with/before nouns and means "such/like this" (thing/object near the speaker), in other words, you need a noun after it.「こんな日」= such a day / day like this. Again, you can't say これ日.