Compelled speech.

You could probably write the law in a way that avoids first amendment implications. For instance, require that people show up at a polling place, but do not require them to submit a ballot.

Voting is political speech. The right to say nothing is as important as the right to speak.

Have an empty spot you can vote for. So they still have to go through the steps and send it in, but they don't actually have to speak/vote on something

You have every right to leave the ballot blank.

Or write Big Bird in.

Voting is a form of speech, so you can’t be compelled to speak or not under threat of punishment.