I have been using Shopify for awhile and i have yet to be able to setup my checkout system properly. I am using Paypal PayFlow and am sincerely struggling to get it all working properly. It is having me close to going back to my old host for my website. I keep getting an error of "Invalid Vendor Account" when i try to do a test checkout either with a credit card or with PayPal itself. I have tried deleting everything and restarting from the beginning but nothing seems to worse. I am very new when to comes to this type of work and coding so i could be messing it up by something simple but i could not know. I am at a loss and have no idea where to go for information. I will gladly hire someone to help me fix this issue and get it working again properly.
I don't think it's a coding issue, you actually have very limited control over the checkout system.
I did a search for that error message and think I found your solution: https://help.shopify.com/en/manual/payments/paypal/set-up-paypal#invalid-vendor-acct
Follow those steps, and if that still doesn't work I would contact Shopify support and let them know you're unable to accept payments.