You're running a command which operates on a range of revisions (usually, from some revision to HEAD) but have not specified the revision that should determine the start of the range.
Previously, arc assumed you meant '$1' when you did not specify a start revision, but this behavior does not make much sense in most workflows outside of Facebook's historic $2 workflow.
arc no longer assumes '$3'. You must specify a relative commit explicitly when you invoke a command (e.g., `$4`, not just `$5`) or select a default for this working copy.
In most cases, the best default is '$6'. You can also select '$7' to preserve the old behavior, or some other remote or branch. But you almost certainly want to select 'origin/master'.
(Technically: the merge-base of the selected revision and HEAD is used to determine the start of the commit range.)