Thanks for letting us know that you're experiencing this issue. Allow me to provide some information about regarding connection issue between PayPal and QuickBooks Online.
Customers are unable to connect with their PayPal accounts is an on-going issue in QuickBooks Online. We have been receiving reports coming from our customers about this issue and our Engineering Team is working closely to get this resolved.
While they do, as a workaround, I would advise you to please login to a Chrome Incognito Window (or private browsing window) and attempt to connect the app.
I'd recommend reaching out to our QuickBooks Online Support so they can add your account information to the list of affected customers. Any progress will be communicated via email.