I'm using both Nostr and Fountain from Android and when I try to connect my Nostr account with fountain I get this error message 
Android Remote Signers like Amber let you sign remotely on mobile from a single app - https://github.com/nostr-protocol/nips/blob/master/55.md
What went wrong with the web extension exactly?
Discussion
Do you have the Amber signer on your phone? If yes can you DM me your Fountain user id (available in the menu) and I'll investigate further.