What part of speech is err?

"Err" (pronounced just like "air") is a verb.

It simply means 'to make a mistake', and when you do that,
you create an 'error'.

There are millions of ways to err in the life of a high school student.
You err if you leave for the bus 15 minutes after it passes your corner.
You err if you tell your sister that she looks particularly fat today.
You err if you leave all of your books home on a school day.
You err if you tell your mother that you never liked her cooking.
Some of these actions are not only errors, but could also be hazardous
to your health as well, too.

Its a verb because you can actually do that word in a movement or somerhing with your body h

Omit means cleave out .