Ukraine wasn't allowed to join NATO for reasons that NATO had issues with corruption in Ukraine.
During President Trump's phone call with Zelensky, he said, and I quote:
"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. Biden went around bragging that he stopped the prosecution so if you can look into it. ... It sounds horrible to me."
It is a fact that Biden stopped Ukraine's investigation into the corruption involving Burisma and Hunter Biden, by threatening the previous Ukrainian President with the withholding of $1 billion dollars of loan guarantees:
"In his own words, with video cameras rolling,
Biden described how he threatened Ukrainian President Petro Poroshenko in March 2016 that the Obama administration would pull $1 billion in U.S. loan guarantees, sending the former Soviet republic toward insolvency, if it didn’t immediately fire Prosecutor General Viktor Shokin.
“I said, ‘You’re not getting the billion.’ I’m going to be leaving here in, I think it was about six hours. I looked at them and said: ‘I’m leaving in six hours. If the prosecutor is not fired, you’re not getting the money,’” Biden recalled telling Poroshenko.
“Well, son of a bitch, he got fired. And they put in place someone who was solid at the time,” Biden told the Council on Foreign Relations event, insisting that President Obama was in on the threat."
Joe Biden's 2020 Ukrainian nightmare: A closed probe is revived
In other words, VP Joe Biden actually helped create more corruption in Ukraine, by forcing the previous Ukrainian President to cover it up, by firing that prosecutor.
President Trump on the other hand, was not only trying to expose corruption in Ukraine, but the corrupt dealings of an American Vice President. Had he been successful at it, President Zelensky probably would have cleaned up much of the corruption in his country, Ukraine would have been admitted into NATO, Joe Biden would have never been elected, and Russia would have never invaded Ukraine.
Just think how it could of been. And now thousands of people are dying, oil is at a record high, as is inflation, and we're on the brink of a nuclear war.