Why am I unable to add PayPal to my form?

Trying to add PayPAl option to my form but it is grey - wont let me add it.

That's because you have an existing payment integration on your form and it's not possible to add more than one to the same form. 

To be able to add PayPal simply delete the current integration, to do it, edit your form in the builder, scroll down until the payment field, select it and click on the trash icon: 

I hope this helps.