Trump and his campaign lied about meetign with Russians, then lied aobut what those meetings were about. It all seems very suspicious to me. I fthey were dogn nothign wrong then why lie about it? So, they investigated.
On NPR they reported that the Mueller report seems to make clear that although Trump and his campaign did not coordinate with Russia in theyr attemtps to alter the election, they did know aobut it and encouraged it. That is what they were hiding. Legally this creaes problems as far as proving an actual crime.
When it comes to Obstruction of Justice. Mueller laid otu ten separte cases to be made for Obastruction of Justice. However the Justice Department of which he was wrokign for has an official policity of nto prosecuting sittign presidents, so he could not file charges himself. He did however lay out a road map for the congress to prosecute (impeach) Trump, or lacking that, for Trump to be prosecuted once he leaves office. If Trump gets reelected the statute of limitations will fun out before he leaves office (at least for the crimes Trump has already committed, new crimes would be another story).
So, perhaps Trump's 2020 campaign slogan shoudl be "relected Trump so he won't go to jail."