It is a criminal offense to threaten or extort others in order to make some form of personal gain (whether financial or for political advantage) from actions that you are legally- and duty-bound to perform. Trump has been extorting people all his life, this was just one more incident.
By withholding military aid from Ukraine, which had already been approved by Congress, as a means of coercing their President into digging up dirt on Joe Biden and his son, Trump was effectively forcing the Ukraine to provide him with a political advantage while withholding Congressionally-approved funds from the Ukraine until such a time as this was done.
The transcript makes it clear enough that this one was akin to a mob-boss shakedown: do this for us, and we’ll continue to offer you support. This particular part was rather telling:
“I will say that we do a lot for Ukraine. We spend a lot of effort and a lot of time… I wouldn’t say that it’s reciprocal necessarily because things are happening that are not good, but the United States has been good to the Ukraine.”
Does that or does that not sound like a veiled threat: we’re being good to you now, but that could change at any moment. Particularly when we come to this:
“I would like you to do us a favor though because our country has been through a lot and Ukraine knows lots about it… There’s a lot of talk about Biden’s son, that Biden stopped the prosecution and a lot of people want to find out about that so whatever you can do with the Attorney-General would be great.”
Really, Donald? Neither Biden nor his son have been implicated in any wrongdoing - that’s all a slanderous allegation leveled by Trump at Hunter Biden’s participation in business dealings in Ukraine, and neither the Ukraine nor the US found that any wrongdoing had been perpetrated on his part. This is all a smear attempt to get Biden’s name put next to words like ‘corruption’ - as you’ll note Trump has done several times, in the hope that his supporters buy into it.
This has absolutely nothing to do with the Office of the President of the United States. It has nothing to do with serving the country. This one’s all about Donald Trump, and his re-election bid. He’s terrified that Biden might gain the Democratic nomination for the 2020 Presidential election, because the polls place Biden well ahead in that particular race, and his previous executive and political experience far eclipse that of Trump. As was done with Clinton, Trump knows that his best shot at winning is to smear Biden sufficiently that independents stay home on election day, meaning that his support from the Republican Base alone is sufficient to re-elect him.
In reality, however, we’re seeing Trump yet again drawing in a foreign nation and using them in an attempt to influence a Presidential election. Doing so as a private citizen was one thing - doing so while President of the United States is a clear and shocking violation of his Oath of Office.
Impeachment is the only possible recourse here.