Partion Samsung T5 on MAc
You cannot partition a volume. You must partition a drive (device). To do so, click on the drive itself, located just above the partition you selected on the sidebar ("Samsung Portable...").

Then, you can partition the disk.

Edit: Back all data you want to keep up and Erase the disk as GPT (GUID Partition Table). Newer versions of DU for some reason don't want to work with MBR partition tables. Choose ExFAT as file system to be able to read and write to the disk from both Windows and macOS, or APFS if you only use Macs.