The transcript shows Trump soliciting aid from a foreign government, asking them to investigate a political rival. That is illegal.
If you read Trump's remarks in the transcript, you'll see he implicates Joe Biden in the situation and asks the Ukrainian government to "look into it".
Not only that, it shows him asking them (Ukraine) to get the US Justice Dept (Barr) involved (“I will get the AG to call you”) aka meddling in order to get the US JD to also go after his political rival - also a very big no-no.