The Secret of the First Pyramids is a mid-length quest chain in Assassin's Creed: Origins that is doable at level 23+. It used to come as a pre-order bonus for anyone who pre-ordered Assassin's Creed: Origin prior to its release, but has since been made available to everyone else.
If you want to play this quest chain, but didn't pre-order the game, all you have to do is to head over to the UbiClub. You can access this from the main menu before loading your game file. You must pay 40 Ubiclub points to unlock it, which should be very easy to get. If you don't already have that, just check out the challenges for Assassin's Creed: Origins, and complete a few of them to get the 40 Ubiclub points you need. These are not points you have to buy with real money!
So that's how to get the The Secret of the First Pyramids quest-chain in Assassin's Creed: Origins, even if you didn't pre-order. Have fun with it!