I was having the same problem with a Samsung washer and dryer. I already have a Samsung TV connected to SmartThings with no problems, so I thought the problem must have been the washer dryer.
I started trying on Sunday, tried all suggestions in this thread about the wifi network settings, resetting the washing machine, everything in every thread I could find and nothing worked. Tried using a new router, tried using the wifi hotspot on my wife’s phone, tried using my wife’s SmartThings app, tried her SmartThings account, nothing worked. Called Samsung on Monday, they tried everything, nothing worked, they escalated to Level 2 Tech Support, but they never called me back.
Now on Wednesday I just got an Android system update. After updating Android it seems like a new version of SmartThings was installed / updated / reset. After that I was able to pair the washer and dryer on the first try. Didn’t make any changes to my wifi setup or the washer / dryer. Using a Note9.
So I’d also suggest try to check if there are any Android updates available. If not, try to uninstall SmartThings, then reboot your phone in between, and then reinstall SmartThings after the reboot. Or, just try waiting a few days, it seems like the one thing everyone has in common is it just started working after a few days.