iPhone app not working and can’t log in

My iPhone and iPad both run iOS 12.4.1. No problems with the app.
You should power down your phone and then restart it. Press the sleep button for several seconds. Then it asks you if you want to power down the phone. After the screen is turned off, press the sleep button to wake the phone up.
What iPhone model?

Looks like your network problem
Ditto, Countryboy is growing on me by the way, every time I hear from him he is making sense.
OP, you need to reset your network on your Iphone, google it if you don't know how, then try uninstall/reinstall of chess app. Then also, how about spend 30 min. removing all the apps you never use but once in a Blue Moon.
The above steps will fix 84.2% of all phone problems that come up. This is proven scientific statistic.

You are welcome, thank you for letting us know all is well. Good on you for not loading your phone down with apps. Hey maybe if I help enough people, chess.com will hire me on to the staff, then I can quit bagging groceries. Can I add you to my list of references?

You can try these tips to fix it.
1. Force quit the app and open it later.
2. Force restart your iPhone.
3. Software update.
4.Use iOS system repair tools such as TunesKit iOS System Repair to resolve it without data loss.

You can try these tips to fix it.
1. Force quit the app and open it later.
2. Force restart your iPhone.
3. Software update.
4.Use iOS system repair tools such as TunesKit iOS System Repair to resolve it without data loss.
This is a thread from 2019
You can try these tips to fix it.
1. Force quit the app and open it later.
2. Force restart your iPhone.
3. Software update.
4.Use iOS system repair tools such as RecoveryTool Fix Recovery to resolve it without data loss.
Your method may be feasible, but this is a post a long time ago.

You can try these tips to fix it.
1. Force quit the app and open it later.
2. Force restart your iPhone.
3. Software update.
4.Use iOS system repair tools such as RecoveryTool Fix Recovery to resolve it without data loss.
Your method may be feasible, but this is a post a long time ago.
2022 same problem

Try the following:
- soft reset the phone
- hard reset the phone
- reinstall app
- update app
If non of this work, gives us a shout, we'll be happy to help - iFix

I can tell you what worked for me, what i did was this signed out and then sign in to iPhone again.
Did you read the date of the post you were answering?
It was from March 2023.