How can i get cu alter?

Cu Alter is unlocked once you finish E-Pluribus-Unum (North America) Chapter. Before you finish that chapter, he is not unlocked in the pool. No matter how many pulls you do, he will not show up.

After you clear North America chapter, he's unlocked and can be potentially pulled from RATE UP banner(edited) or story banner.

Your best bet would be next year's Chaldea Boy's Collection banner, around March.

Also, Jeanne Alter cannot be pulled from normal pool. She can only be pulled during her banner. And you just missed that banner from a month or so ago. The next time she becomes available would be in a year and a bit during 7 Counterfeit Hero rerun, which is in 2020, if NA follows JP schedule strictly.

If you want Cu Alter so bad I must make one thing clear
He'll not show up on any event banner (unless he's featured in it, like Berserker-class banner or singularity pickup summon), you must roll on story summon

You can try your luck for him during the GSSR Berserker banner as you’ll have a 1/3 chance of getting him. It is a paid gacha however so you’ll have to spend money.