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

Reading the SuSE section titled "2.4 Changing the Size of an XFS File System" seems to use the /var/opt (Mounted on) instead of the /dev/... (Filesystem) that you are suggesting.  Maybe the command should be:
sudo xfs_growfs -d /var/opt

If I take a snapshot before I increase the drive, can I revert it back to that snapshot if there is any issues or will that cause issues on retain?

Nancy - do you have the databases and indexes on the same server?  If so, then this won't be an issue.  Bring Retain down, take a snapshot of the entire system before the expand.  If the dbs and indexes are not part of the snapshot, then make sure to disable archiving and deletion jobs before the expand.  The less change to the system before or after, the better it will be.  Once you do the expand you can verify that all is well with the system and if so, re-enable archiving and deletion jobs.  -  Pam

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