The error indicates that tokio isn't configured right.

tokio has to be built with the unstable flag.

There is a file ".cargo/config" which sets this flag.

If you set RUSTFLAGS you will clobber this setting, so you have to add "--cfg tokio_unstable" to RUSTFLAGS if you override it.

Gossip has a Cargo.lock which fixes in place the versions of all the dependenices so you can't be running some new tokio that breaks.