I’d like to check out a few things with you here directly to see if we can ascertain the root cause of this behaviour.
Can you confirm for me that your store is on a paid plan already and you have access to the checkout area?
If so, can you give me the exact steps you use when you see this behaviour triggered?
Outlining the exact steps you take from your storefront or Shopify admin will allow us to understand your process and attempt to recreate these circumstances.
The more information you can give us here the better we will be able to understand and help out.
We wouldn’t be able to email you out from here for support, so I’d like to eliminate a few potential causes before we may need to direct you towards account support.
Your next best bet would be to try making a test order there, after disabling your password page temporarily.
I’d also double-check to ensure your PayPal account settings match those of your Shopify store in terms of country and currency especially.
I infer from your reply here that your friend was testing using their own PayPal account, but if not, ensure you’re using a different PayPal account than the one you have attached to this store for payouts.
Let me know if you continue to experience this issue after making the checks above!