Mail-in ballots, with envelopes to open and signatures to check, have always taken more time and manpower to tabulate than in-person votes. And this year, there are simply more mail-in ballots to count than in previous election years.
Pennsylvania is one of three key battleground states — along with Michigan and Wisconsin — with restrictions on when mail-in votes can be processed. For Berks County, that means that hundreds of election workers began opening mail-in ballots at 7am this morning, and election officials are optimistic that their results can come in shortly after polls close. But in 7 other Pennsylvania counties, mail-in ballot processing won't start until tomorrow, meaning that hundreds of thousands of votes will need to be counted in the days following the election. If the race in Pennsylvania is as close as it was in 2016, we simply won't know who won until enough of those votes are tallied up.
That said, it's still possible that we will know who won, even if we don't know who won in Pennsylvania. Not all battleground states are slow-counting states. Florida, for example, allows election offices to process mail-in ballots as they come in, up to 22 days before the election, meaning that there could a a close-to-complete county by midnight. If enough of these states swing a certain way, it's possible that one candidate could get to 270 electoral votes before the results from Pennsylvania, Michigan, and Wisconsin come in.
It's also important to note that a clear partisan divide between mail-in and in-person voters could mean that initial unfinished results could appear more favorable for one candidate than the end results, so pay close attention to which vote is being reported. States that process mail-in ballots early should have those results reported first, so initial returns in states like Florida, Georgia, Ohio, and North Carolina might lean heavily Democratic, then become more Republican as the in-person votes are tallied. In Pennsylvania, where in-person votes are likely to be counted first, you could see the opposite effect.
If we don't know who won by the end of tonight, or by the end of tomorrow night, it is not the result of some grand voter fraud scheme, and it isn't the out of the ordinary. There has never been a presidential race in history in which all votes are counted in election night. Be patient!