Are you using astral.ninja? In nostr, events are of different types, some of them are notes, which can be either short (280 characters or less) or long (not sure if they are limited. In astral.ninja, while you are typing, you get the number of character left, when they are 25 or less, inside a little orange circle under the note. Once you reach more than 280 chars, the excess number appears in red, and a little "button" appears beside it to switch between a "short" and a "long" note. A LN invoice is a little longer than 280 characters, so you need to turn the switch to "long". Hope it helps. Let me know if you won't success and I'll take some screenshots for you.