This is because for the moment my app only supports keysend. Contact your wallet provider and ask them to support the only truly peer to peer lightning protocol that doesn't rely on censorable things like DNS
With that said my app will eventually support zaps as soon as an acceptable solution to the metadata "messages/boostagrams" is solved
For now the only wallet solution is alby, though I hear that coinos has finally implemented keysend support but I haven't tested it yet.
Zaps use lnAddress which uses a DNS reliant server in the backend to create an invoice on your node's behalf. So invoice are still required for zaps, it's just that they are created for you on the backend.
lnAddress also limits the amount of characters to around 200.
Keysend on the other hand just sends sas directly from node to node with no middleman in between. Even works with Tor.
Also keysend supports the TLV record which gives crucial metadata about the boostagram including allowing for much larger messages.
See