Thanks for using Cloudera Community. The Error  indicates your Team is using CDH v5.x using HBase v1.x. In the concerned release, We have limited choices to ensure "hbase:namespace" Table Region is assigned correctly as shared below:
(I) Easier Yet Not-Always-Successful Option:
1) Shutdown HBase Service
2) echo "rmr /hbase" | hbase zkcli (From HMaster Node)
3) Start HBase Service
4) sudo hbase hbase hbck -fixAssignments
(II) Review whether any of the "hbase:namespace" Table Region files are reporting MissingBlockException. If Yes, We need to fix the MissingBlockException.
(III) Possible for the RegionServer Logs (Wherein the "hbase:namespace" Table Region is being assigned) would offer additional insight into the reasoning for the RegionServer being Unable to assign the "hbase:namespace" Table Region successfully. Your Team may review the concerned RegionServer Logs as well.
There may be Other Options, yet such Inconsistencies are better handled in HBase 2.x  & your Team should plan for using HBase v2.x soon.