Problem updating October

I’m using October 3.6.10 and trying to update to the last version. Also, I’m using the plugin RainLab.User version 2.1.0, which could be updated to version 3.1.0.

I tried to update with:

sail composer update

But I got this error:

Your requirements could not be resolved to an installable set of packages.

Problem 1
- Root composer.json requires rainlab/vanilla-theme ^1.0, found rainlab/vanilla-theme[dev-master, dev-next, v2.0.0] but these do not match your constraint and are therefore not installable. Make sure you either fix the constraint or avoid updating this package to keep the one present in the lock file (rainlab/vanilla-theme[v1.0.3]).

Any clues to solve it?

Best regards.