ORA-15167: command requires at least 3 failure groups; found only 2 - ASM

SQL> ALTER DISKGROUP DG_NORMAL CONVERT REDUNDANCY TO FLEX;

ALTER DISKGROUP DG_NORMAL CONVERT REDUNDANCY TO FLEX
*
ERROR at line 1:
ORA-15032: not all alterations performed
ORA-15167: command requires at least 3 failure groups; found only 2

Solution:-
To migrate a DG to FLEX DG following is required
1> only NORMAL or HIGH redundancy DiskGroups can be migrated to FLEX diskgroup
2> Diskgroup must be mounted in RESTRICTED mode.
3> Compatible.asm and  Compatible.rdbms of the diskgroup should be 12.2.0.0.0 or higher
4> Requires at least 3 failure groups

Since there is only 2 failgroup its not satisfying the prereq to convert the diskgroup to FLEX
Modify the Diskgroup to have minimum 3 Failgroup and try again

You can Check the Failgroup of a disk belonging to a diskgroup as follows

SQL> select di.DISK_NUMBER,di.NAME,di.FAILGROUP, dg.name from v$asm_disk di,v$asm_diskgroup dg where di.GROUP_NUMBER=dg.GROUP_NUMBER and dg.name='DG_NORMAL';