The contains a series of sections designed to prepare students for the ACT?

That sentence is incomplete. To make it a complete and grammatically correct sentence, you need to add a clause that describes what the unspecified thing *contains*. For example:

* "The workbook contains a series of sections designed to prepare students for the ACT."

* "The course contains a series of sections designed to prepare students for the ACT."

* "The prep book contains a series of sections designed to prepare students for the ACT."

The missing element is the noun indicating *what* has the sections.