Instead of dealing with the folder from within your OS File manager application, move your other folders and files into a freshly created Drobo Share, then erase the share with the problem folder. I have detailed the steps below.
On occasion, I have had a file or folder in a share that will simply not be deleted, no matter what. Frustrating, and has happened more than once.
My solution - after all else has failed? Within the Drobo Desktop, create a new share to replace the share with the problem. Then in you file manager (Finder in macOS, or File Explorer in Windows) copy all the good folders and files from the problem share [BAD_SHARE] into a newly created share [GOOD_SHARE], but don’t copy the problem folder(s) or file(s). Now that you have duplicated your share minus the problem folder, just ERASE [BAD_SHARE]. As a final step, you can rename [GOOD_SHARE] to the original name of the deleted [BAD_SHARE], if desired.
Easy to accomplish, but sometimes, not so obvious.