Obligatory: Muun is an on-chain wallet that uses a security model that is outside the specification for the Bitcoin protocol to create the illusion of LN transactions
Withdraw -> Lightning address -> paste the invoice there and specify custom amount
It seems there is no way to get a "lightning address" from muun wallet, you can only create a lightning invoice where you specify the amount of sats you want to recieve. I created an invoice and it worked perfectly.
You likely put your invoice in the lightning address field rather than the invoice field. That's the only way that message would have appeared. Muun does not support LNURL or lightning address.
I just tested with both Strike and Muun and it succeeded.
SN does not support 0 amount invoices currently. When using Muun, you'll need to specify the amount when creating the invoice using "invoice settings."