why do i get "we can't complete your purchase' message?

[AM2017]

I have run a Shopify e-commerce site for 3 years now (linked to Paypal) - with no major issues

However yesterday doing routine testing (where i run dummy payments)

I got a :

"We're sorry. We can't complete your purchase" message

This happened TWICE with 2 Credit cards

BUT WORKED when I used Debit cards (tested twice)

Anyone know what is happening?

Is this linked to TLS1.2 deadline glitch - my connection is 'OK' (tested)

Needless to say I maybe losing sales!

Thanks

Mano