Your Retain index directory is located at /var/opt/beginfinite/retain/index/
We hope that the person that installed Retain allocated a separate disk for Retain.
Run this command to find out how your Linux OS disks were allocated:
df -Th
Did that person make any part of the above path, mount to a separate disk than what was used for the Linux OS root partition at / ?
Like    Filesystem: /dev/sdb1 , Mounted on: /var
Here is SuSE's SLES 12 chapter on resizing filesystems:
https://documentation.suse.com/sles/12-SP5/html/SLES-all/cha-resize-fs.html

This is the filesystem that I want to increase /dev/mapper/beginfinite-opt  xfs  750G 564G 187G 76% /var/opt

What Is the command?  Is it ...

sudo xfs_growfs -d /dev/mapper/beginfinite-opt

data size unchanged, skipped when I run sudo xfs_growfs -d /var/opt
 

Not sure why?  df -Th  results below.
Filesystem                  Type      Size  Used Avail Use% Mounted on
devtmpfs                    devtmpfs  4.0M     0  4.0M   0% /dev
tmpfs                       tmpfs     9.7G   16K  9.7G   1% /dev/shm
tmpfs                       tmpfs     3.9G  9.2M  3.9G   1% /run
tmpfs                       tmpfs     4.0M     0  4.0M   0% /sys/fs/cgroup
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /.snapshots
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /boot/grub2/i386-pc
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /root
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /srv
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /opt
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /tmp
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /usr/local
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /boot/grub2/x86_64-efi
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /var
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /home
/dev/mapper/database-mysql  xfs       100G   45G   56G  45% /var/lib/mysql
/dev/mapper/beginfinite-opt xfs       750G  566G  185G  76% /var/opt
tmpfs                       tmpfs     2.0G   60K  2.0G   1% /run/user/468
tmpfs                       tmpfs     2.0G   40K  2.0G   1% /run/user/0

Am I running the correct command?

data size unchanged, skipped when I run sudo xfs_growfs -d /var/opt
 

Not sure why?  df -Th  results below.
Filesystem                  Type      Size  Used Avail Use% Mounted on
devtmpfs                    devtmpfs  4.0M     0  4.0M   0% /dev
tmpfs                       tmpfs     9.7G   16K  9.7G   1% /dev/shm
tmpfs                       tmpfs     3.9G  9.2M  3.9G   1% /run
tmpfs                       tmpfs     4.0M     0  4.0M   0% /sys/fs/cgroup
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /.snapshots
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /boot/grub2/i386-pc
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /root
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /srv
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /opt
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /tmp
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /usr/local
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /boot/grub2/x86_64-efi
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /var
/dev/mapper/system-root     btrfs      61G   19G   40G  32% /home
/dev/mapper/database-mysql  xfs       100G   45G   56G  45% /var/lib/mysql
/dev/mapper/beginfinite-opt xfs       750G  566G  185G  76% /var/opt
tmpfs                       tmpfs     2.0G   60K  2.0G   1% /run/user/468
tmpfs                       tmpfs     2.0G   40K  2.0G   1% /run/user/0

Am I running the correct command?

Unfortunately this is a SLES issue and no longer a Retain issue.  As no entitlement for SLES comes with Retain, please either open a case directly with SUSE (scc.suse.com/support) or post in their forums (https://forums.rancher.com/c/suse-product-topics/24).  They can assist you with this.

Pam