ORA-15000: command disallowed by current instance type

ORA-15000: command disallowed by current instance type

As the error itself suggests we are not using the right instance type. It occurs when we try to adminitrer the ASM storage system from RDBMS instance such as adding disks,dropping disks,etc
Solution
Connect to the ASM instance and execute the command connecting 'as sysasm'
for instance
$export ORACLE_SID=+ASM
$sqlplus / as sysasm
SQL>ALTER DISKGROUP DATA ADD DISK '/dev/rhdisk391' REBALANCE POWER 11;