The reason why you fail to use WhatsApp Transfer is:
You have enabled two-step verification.
In order to use WhatsApp Transfer successfully, please disable two-step verification by the following steps.
Step 1: Go to ‘Settings’ on WhatsApp.
Step 2: Click ‘Account’ settings.
Step 3: Select ‘Two-step verification' settings.
Step 4: Tap on ‘Disable’ and confirm ‘Disable’.
Step 5: See the following interface, which indicates the two-step verification has been disabled successfully.