Before you are able to cash out, you will need to verify your account. This can be done by verifying your email address and phone number, as well as your cash out email address which must be the email address associated with your PayPal account.
To verify your phone number you need to enter the number with the country code and no hyphens or spaces etc.
Note that your phone number/email address information is collected for verification purposes only. This information will not be used for marketing purposes in any way, unless you explicitly opt in for marketing.
Please visit https://www.prolific.co/participant/trust to complete the verification process.