When you go into settings to add a relay make sure it’s in the proper format “wss://nostrplebs.com” for example. Also, some relays are paid, meaning you have to go to the website and pay sats to be granted read and write access. Here is a list of every single relay. Feel free to take a peak at mine as well, they work well but most of mine are paid. https://nostr.watch/relays/find