In order to fix the problems, I recommend you delete your version of WhatsApp and download and install the latest one from App Store. After that, in order to register, you will need to enter your full international phone number (including the country code). If you don't know the country code, you can select it from the provided country list. Next, you will need to wait for the confirmation message. Once you receive it, enter the 6-digits code in the verification screen of WhatsApp. If you have problems, use the contact information from the official web page to get in touch with the support team.