Hello, in my spare time when I'm I'll be on my mobile phone and make new characters and most of the time now when i do subclasses, feats and sub races don't appear. I only have access to the handbook and even those aren't appearing. Example I'm currently playing an assassin rogue and about to multiclass into fighter and i wanna become and arcane archer but when i try to make it and read about it, I can only choose champion. I don't wanna make that character a champion.

There are three ways to get access to non-free content: buying it on here, having it shared with you and replicating via homebrew.

Buying

You will need to buy from the marketplace store on here. If you have bought the PHB and not seeing PHB contact please check your entitlements are in sync by going into your account and checking under marketplace there. If you're not it gives the option, just click and try again. If that doesn't work raise a support ticket by using the Contact Us page found at the bottom of any page.

Sharing

You must make the character in the campaign that has sharing enabled.

Homebrew

If you have the content elsewhere you can recreate most things using the homebrew tools. To use homebrew on your sheet you need to turn Homebrew on - on the home page when editing the character there is a toggle for homebrew - click it to turn on (red).

The Champion is the only free subclass available for Fighters as it was the only subclass that was ever published for free by WotC, the company that actually publishes D&D. The only free subclass for Rogues is the Thief. Since your character already has the Assassin subclass, that means you must have had access to the PHB (or at least that one subclass) at some point. If you have not purchased the PHB from this website, or at least that subclass from the PHB, then that access was most likely gained through content sharing in a campaign by someone who has purchased either that book or that subclass.

If you have purchased the PHB, then click here and make sure that your entitlements are in synch:

https://www.dndbeyond.com/account/marketplace

The Arcane Archer subclass can be found in Xanathar’s Guide to Everything, you will either need to purchase that book, or at least they subclass from that book to unlock it.

I'm having a similar issue. I have a fighter that I'm trying to make an Echo-Knight. I found a homebrew someone made of the echo-knight and added it to my collection, but then when I go to add it to my fighter, all that shows up is Champion and Gunslinger. I have my allow homebrew toggle turned on (he possess many homebrewed items) and it's showing up in my collection of homebrew. I just can't add add it for his martial archetype.

That's why I'm using the homebrew option, so it's not affected by the different versions. I got it figured out though, so it's fine.