To be able to use PayPal for Virtuagym payments, you will need a PayPal merchant account. This can be created on https://paypal.com/.
As soon as you've created an account, you go to Business Settings > Payments in Virtuagym. Here, under available payment providers, you can tick the box PayPal. Under "PayPal merchant account", enter the email address that is attached to your PayPal account. Save it.
Now you can go to your financial app, find any invoice and under options at the top right of your screen, click on "email including pay-link". This will send the invoice as a PDF to the client, with the link to PayPal to immediately pay the outstanding amount. If you'd like to set up your own email, you can get the payment URL in the invoice under "Pay".
When the payment of the client goes through, this is automatically updated in Virtuagym.