"Undervoting"—selecting less than the maximum number of choices available on a ballot—is a common (and legal) practice among American voters. Sometimes it's intentional (in protest, or as a thoughtful abstention), sometimes accidental.
According to the Federal Election Commission, in 2016, 136.7 million Americans voted for president, but only 131.7 million voted for a local House candidate. The same pattern is observable in 2012, 2008 and 2004.
The Washington Post found 1.7 million voters (or 2% of the total vote) abstained from voting for a presidential candidate in 2016.