Cannot Activate WhatsApp on Oppo A15s Due to Verification Failure

I am using an Oppo A15s, and for about two weeks, I have been unable to use the official WhatsApp application on my phone. The download sometimes completes and the application installs, but it does not work at the phone number verification stage, and I cannot activate my account. I would like to receive clear and effective technical support so that WhatsApp can be properly installed and my number can be verified on my Oppo A15s, or any other concrete solution that will allow me to use the application normally again.