PayPal Invoicing: Unable to integrate

My group already has a PayPal account so I don't need to create one.  It keeps asking me to create an account with an email.  I actually log into our PayPal with a user name not an email address. 

You can enter your email address, but then select the 'Log in' option on the second step:

As far as I understand, you need the 'Access APIs and developer support site' permission enabled for your PayPal user. This is something that is managed by the main PayPal account user.