Usually, です is a polite copula, similar to だ but more polite:
それはリンゴだ That is an apple
それはリンゴです That is an apple (polite)
But です can also be a politeness marker added to adjectives:
あかい is red
あかいです is red (polite)
When it's a politeness marker, です doesn't inflect for tense:
あかいです is red (polite)
あかかったです was red (polite)
The adjective before it already inflects for tense.
This is a relatively recent innovation in the Japanese language and not too long ago was considered unacceptable. Some people still try to reword things to avoid it, but it's probably caught on because it filled a useful gap in the language: making adjectives polite, like です with nouns or 〜ます with verbs, but without going as far as 〜うございます.
However, verbs already have a way to make them polite:
つかれた (past)
つかれました (past, polite)
So there's no motivation to start saying *つかれたです, and there's no reason for people to start treating it as an acceptable part of the Japanese language. As a result, it's ungrammatical.