How old is the iPad and what iOS version are you on?
requirements from Zwift.com:
iPad® Air or higher, iPad® Pro or higher, iPad mini® 2 or higher, or iPod 6 or higher *(this should probably be iPad 6, not iPod 
)
iOS® 9.0 or higher
How old is the iPad and what iOS version are you on?
requirements from Zwift.com:
iPad® Air or higher, iPad® Pro or higher, iPad mini® 2 or higher, or iPod 6 or higher *(this should probably be iPad 6, not iPod )
iOS® 9.0 or higher