Why "should backup" needs a backup plan

What kind of error is it?

Modal verb agreement error

Why do people make this mistake?

The phrase "should backup" is incorrect because "backup" is being used as a verb, which means it should be "back up" instead. "Should" requires a base form of the verb, and "backup" as a single word is a noun or an adjective, not the base verb form.

What is correct?

The correct verb form is "back up" when used after a modal verb like "should". "Should back up" is the proper construction for expressing the action of creating a backup.