As noted by @Xenopus on the Wayfarer Discussion Discord, it is probably related to this known issue: Wayfarer Nominations Blocked in Pokémon GO v.0.387 and it can be fixed by logging in and out (sometimes a couple times) so try that
I’ve subbed using both the Web and in-Game today, and both worked for me without the need to sign out or in in the style of the Hokey Kokey…. But will keep the workaround in mind if I get the issue.