Can't create diskgroup in Oracle 19c
It seems you are in RDBMS instance
to check use the following query:
select value from v$parameter where upper(name) = 'INSTANCE_TYPE';

or
show parameters instance_type

If you get RDBMS value, go to operational system shell and check the SID of ASM instance in /etc/oratab
cat /etc/oratab

After cat the file, you will get something like it:
+ASM:/u01/app/19.0.0/grid:N

The rows are delimited by :. The first field is the SID (in my case +ASM), the second field is the GRID_HOME (also ORACLE_HOME)
You can use oraenv to set the variables needed to access the ASM instance or you can set the variables manually with export command.
With oraenv:
. oraenv

after issuing the . oraenv command, type your SID
or export the variables manually
export ORACLE_SID=+ASM
export ORACLE_HOME=/u01/app/19.0.0/grid
export PATH=$ORACLE_HOME/bin:$PATH

NOTE: to create asm diskgroup you need access the instance with sysasm role
sqlplus / as sysasm