The National Express counters seem to be closed at many of the UK airports currently, I can only assume this may be linked to when the Covid pandemic started?
if you’re not comfortable booking online then you can purchase a ticket directly from the driver on the day, providing there are seats available on the bus journey you’re planning to take. It’s probably advisable to check the bus timing you’re considering via the website at Nationalexpress.com first to ensure there’s seat availability in order to avoid any disappointment.