Born of bitter experience 35% of European countries apparently don’t allow absentee ballots even for those out the country. 74% ban absentee ballots for those who are in the country and most of those who allow it require a photo ID.
The rules the US courts permitted at the last moment were extreme by historical standards and invited cheating. The courts have admitted cheating occurred but put the burden of proof on the Republicans to show it was extensive enough to make a difference....but then they denied discovery until there was enough evidence. A catch 22 situation.
It’s unclear whether there was enough fraud to change the election, setting aside the mega fraud perpetrated by the media which left half the voters believing the Russian fraud is real. That certainly changed the election...as did the legal use of absentee ballots by the Dems eg in Pennsylvania.
All these things obstructed a free and fair election....strongly indicated by the Dems loss of house seats which shows the underlying trend. If you live in a Democracy you accept the will of the people....but you have to be sure that’s what you are measuring. Any honest individual has to admit the Presidential election fails that test.