Hello Alan,
The information you have provided is not really detailed enough to determine where your problem lies. But let's start by looking at some basic numbers.
Your destination site (remote site) has a 21.5Tb Pool with 16.5Tb as free. If that is all replication data it indicates that 5Tb has replicated from a 4Tb volume on the source. It may be you have more than one 4Tb volume from the source replicating which would explain this difference. Without more data from both systems and their volume usage stats it is hard to provide more information.
The other explination could be you have multiple snapshots you are attempting to replicate. You need to remember that for every volume you are replicating there are internal snapshots also being replicated to maintain consistency. I suggest you review the SMU Guide starting around page 117. https://support.hpe.com/hpsc/doc/public/display?docId=a00017707en_us
Regardless, on the destination array you have the Pool configured to overcommit (Pool overcommit is set to True). Please review what having this feature provides you and understand some of the boundries of having overcommit enabled. Using the SMU Guide you can find this information starting on page 24.
Your error message might have occurred as you passed the Low Threshold. This is normal and if you review the system space use and have enough space for all the volumes + internal snapshots you should be good.
Cheers,
Shawn
I work for Hewlett Packard Enterprise. The comments in this post are my own and do not represent an official reply from HPE. No warranty or guarantees of any kind are expressed in my reply.