From WhatsApp's FAQ page:
For the following platforms, you can no longer create new accounts, nor re-verify existing accounts. However, you'll be able to continue using WhatsApp:
- Android versions 2.3.7 and older until February 1, 2020
Note: Because we will no longer actively develop for these platforms, some features may stop functioning at any time.
So if you're trying to reverify your account, above is the reason why it won't work.
I remember when WhatsApp stopped supporting some BlackBerrys, some were able to change the date of the phone to a few months earlier, and WhatsApp worked! So go ahead and try that. No guarantee that it's going to work.
If you are still having trouble, contact WhatsApp support for further assistance.
A friend of mine had the same issue, he emailed WhatsApp, and they replied that it was an issue on their side and that it's fixed now.
If it's not fixed on your side, restart your phone and try again.