Click and Drop account just won't integrate

[JenniferMBOS]

Can anyone help?

My store is 3 years old and i now have a royal mail OBA account, i have easily linked my etsy and not on the highstreet stores to the royal mail click and drop account but the shopify just won't integrate. 

When i enter in my store name it quickly flashes up with the shopify logo and then all of a sudden goes to a white Forbidden 403 page. When i click in the box it says unsecure.

I really need to not be typing out all my customers address manually into the royal mail account as its a nightmare and it works seamlessly with etsy. 

Thank you in advance 

Jen