Cannot resize partitions

I think you misunderstood the instructions. Grab the sizing gadget in the lower right corner of the Server volume and slide upwards until you have reduced its size by the amount you want to add to the TM volume. Click on the Apply button and wait until the process finishes. Then select the TM volume by clicking anywhere inside its rectangle and click on the Delete [-] button to remove it. Then click on the Add[+] button to add a new partition in the free space. Click on the Apply button and wait until the process finishes. You should now have a resized volume for your TM backup. Be aware that this process will remove your existing backup, so you will need to reconfigure TM and start a whole new backup.

Not that I'm aware of. Were you able to drag the top of the bottom volume upwards that would destroy the data, and DU will not allow you to do that except by purposefully removing the partition. This is to prevent accidental data loss.