I had this issue with my Cassandra 3.11.3 cluster.
'CassandraRoleManager skipped default role setup: some nodes were not ready'
Actually i didn't raised one of my nodes. The status was:
-- Address Load Tokens Owns (effective) Host ID Rack
UN X.X.15.141 279.12 KiB 256 49.1% 65b80558-219d-478b-ad24-083a79d78348 rack1
UN X.X.15.140 235.39 KiB 256 49.2% 0efb432f-0789-455f-op24-29c6e2cce9ac rack1
UN X.X.15.142 287.49 KiB 256 49.6% 3558e3f0-717d-783UI-a22c-6891a3bd427f rack1
DN X.X.15.144 ? 256 52.2% 8e8cdbac-9088-3421-963d-4857a78087d3 rack1
To solve it, i used the nodetool with 'removenode' method. Reference
Removing this node solved the problem for me.