There are some reasons why a datastore can not be removed. Sadly the error description is not very helpfull.
One reason can be that the esx host has coredumps on that datastore. To check that you have to connect to the host and use
esxcli system coredump file list
you will get some output like this:
Path Active Configured Size
------------------------------------------------------------------------------------------------------------------- ------ ---------- -----------
...
/vmfs/volumes/5582db59-64ef7f8d-2c50-0025b501003d/vmkdump/A046FC9D-3156-E111-0001-0000000000DF-6329204736.dumpfile true true 6329204736
/vmfs/volumes/55f95b48-be47d009-3f25-0017a477000c/vmkdump/A046FC9D-3156-E111-0001-0000000000AF-5084545024.dumpfile false false 5084545024
To extract which datastore is yours, you can have a look at /vmfs/volumes, there you find softlinks with the datastore names to those cryptic names.
After the identification you can deactivate and remove the coredump file with:
esxcli system coredump file remove -u
esxcli system coredump file remove -F -f /vmfs/volumes/5582db59-64ef7f8d-2c50-0025b501003d/vmkdump/A046FC9D-3156-E111-0001-0000000000DF-6329204736.dumpfile
If this is not working, than the reason would be, that you are not on the hosts, that this coredump file created. To identify the correct host see https://kb.vmware.com/s/article/10051
If no coredump file is the reason, that re removal is not working, it could still be, that the host is writing his syslog files to that datastore. In this case you have to give the esx host a new syslog destination and restart the host. Then you should be able to remove the datastore.
