First, there are people (called Amillennialists) that don’t believe in the Millennium (a thousand-year reign of Christ), but the Bible clearly says that there will be a thousand-year reign of Christ.
The Book of Revelation clearly says the Tribulation Period martyrs will live and reign with Christ a thousand years (Rev. 20:4-5).
One thousand years is a Millennium.
Amillennialists explain this away by saying the Book of Revelation is allegory.