Republicans in general don't want vote by mail because they know that it is overwhelmingly popular in urban democratic strongholds. It's popular in democratic strongholds in part because it's viewed as practical where voting lines tend to be longer, there are less polling stations per capita, and the poor and elderly often have a difficult time getting to polling stations, particularly in black and brown communities. The arguments about voter fraud have been proven false (over and over again) so it's nothing but a democratic voter suppression tactic.
Every state has different rules. The west coast tends to do this well but in most states you can request an absentee or mail-in ballot.
Learn about vote by mail this election cycle.
It's important to vote early so that you can do your best to ensure your vote is delivered on time and will be counted. The slow-down by the post-master general is about mail-in ballot suppression. It's also important that you follow the instructions very carefully when filling out mail-in ballots so that your vote isn't discarded - have the ballots signed by a witness, sign and date properly, fill in the circles properly, etc. Sometimes it's made intentionally difficult.