bundle2 required
Old clients may see the following error when cloning from new servers:
$ hg clone https://example.com/repo requesting all changes abort: remote error: incompatible Mercurial client; bundle2 required (see https://www.mercurial-scm.org/wiki/IncompatibleClient) [255]
This error means your client isn't using bundle2, which is a new wire protocol format added in Mercurial 3.5. To make the error go away, upgrade your Mercurial install to the latest stable release.