RAID5 with ppl consistency policy can’t be grown without more steps.

Stop the array:

# mdadm --stop /dev/md0

Assemble the array with no-ppl:

# mdadm --assemble /dev/md0 --update=no-ppl /dev/sdg1 /dev/sde1 /dev/sdh1 /dev/sdf1 /dev/sdc1

Add the new drive:

# mdadm --add /dev/md0 /dev/sdd1

Grow the array and set consistency policy back to ppl :

# mdadm --grow /dev/md0 --consistency-policy=ppl --raid-devices=6