Hello. I just went through a very frustrating process to try and add Steam authenticator to a new device after removing it from an old one. Removing from the old device seemed to be pretty straight forward, but I seemed to run into a weird issue that nothing else seemed to touch on. I saw another post where people recommended to make sure Date and Time were correct on the device, and they also said to make sure you're receiving the code for the correct account, but I made sure of all of that. And I had just installed the app, so the app was already up to date. The post also linked to other Steam articles that all seemed to not give any input on the exact issue I was having.

Here is the link:
https://gtm.steamproxy.vip/discussions/forum/1/458605578275947461/

But here was my issue. I was trying to add a number to an alternate account since the old number associated with that account was no longer mine. When adding a number to an account, it sends a link to the email associated with that account. I didn't have the email logged in on my new phone only because I didn't feel like it, and the old device was in arms reach, so I was opening the link on my old phone, and it was sending me an SMS code to enter into the authenticator, and it would advance the process on new phone, but when I went to use the SMS code, it kept giving me an error that read, "The SMS code was not what was expected..." I tried this a few times. But I also noticed that when I was opening the link on the old phone, it was bringing up the Steam authenticator on the old phone as well as progressing the authentication process on the new phone at the same time, so I tried using the SMS code on the old phone as well to see if that would work, but this gave me the same error.

I also noticed that each time I used my old phone to click on the link that was being sent to my email, when I would tap that link, at the same time as it was opening the authenticator on my old phone to enter an SMS code, progressing the authenticator on my new phone, it was also sending me a different unique code through email that I had no place to input since the app was only bringing up a number pad to type in a code, since the SMS codes are numbers only, and the email codes usually have a few letters.

So, long story short, I added the email for the account to my new phone, added the number to the account using the new phone, opened the link to confirm I was requesting to add a new number to the account on the new phone, and then finally was able to enter the SMS code and make the new phone the authenticator for that account. I'm assuming the code that was being sent to the email after opening the link on old phone was superseding the SMS code, even though nothing I could see was asking for this code. Same thing was happening when I would try to add the authenticator without a phone number and just using the email.

I don't know if this is something that should've been apparent to me, or if it's bad app design, but to me, if I click a link somewhere else other than the phone that was making the initial request, it should just advance the process on the intial device, rather than open Steam on old device, advance the request on inital device, and send another code you can't even use to your email. I feel like a lot of other apps do this with no issue. And like I stated above, I don't think I've seen anything, that says that you must open the link on the same device as you are trying to add the authenticator on, or adding the new phone number to the account on.

Hopefully this helps somebody. Or maybe this has worked for somebody in the past, but it didn't register until now what was happening.

TL;DR, when adding an authenticator to a device or phone number to an account, make sure you can open email links on the same device as you are adding the authenticator or it will give you an error.