Yeah, Apple's Disk Utility/Partition tab is a bit stubborn sometimes.
In Disk Utility, you want to make sure you have the
device selected, then the partition tab.
That will show the partitions, as you have them set up. Click on the one you want to split.
Then, click the (+) under the partition layout.
That should show in the graphics that the partition split. Click on the "B2" partition, and change the name to something else (just to modify more than just the partition configuration).
You can ALSO change the size of the split partition, if you have a need to make the split something different than just one half of the original. Just change the size box to whatever you might need. I use that function to make bootable installer partitions, typically 8GB in size. (Or just leave the size as is, all depends on your needs.) But, just reminding you that you don't have to leave the size as the split makes it.
And, when you are ready, click the
Apply button.
The re-partition process can actually take a bit of time to complete, sometimes several minutes. I have been fighting with a PCIe SSD M.2 card, usually extremely quick, that a simple split like you want takes nearly 10 minutes to complete. (I have decided that m.2 card is recyclable
If that doesn't work, where in that process do "things" stop?
