What nostr client? I use zeus and got a self-custody lightning address, super simple.
Discussion
Tired on Damus and primal
I think you are pasting in an ln invoice address not an LNURL. LNURLs are in the format like this: makeasnek@zeuspay.com (my lnurl). I'm not sure if muun supports it.
If you go in Zeus to settings -> lightning address, you can get one there. Sorry I can be of more help on the muun end I've never used it, maybe somebody else can help though
Oh yeah, Zeus is the exception, they use a pretty interesting solution.
I think other wallets don't do it because it's pretty complex and also probably because now that we have bolt12 (read: lightning v2) the new offers feature may (should) become the successor.
The tech is improving.