Issue
So you’re reformatting a hard drive and for whatever reason cannot create partitions because the partition option in Disk Utility is greyed out.
Environment
Platform/Operating System: macOS Mojave
Solution
Go to Disk Utility, then click the View button on the upper left-hand corner and select “Show All Devices“. Now select the parent hard drive you want to partition. Give it a name, select a Format – I usually choose Mac OS Extended (Journaled). Finally, select a Scheme – I typically use the GUID Partition Map scheme. Finally, click Erase. After which, you should be able to select your newly created volume and create partitions – Partition icon no longer greyed out.
Conclusion
I don’t claim to be an expert on hard drives and partition schemes but this is what worked for me.
Important Note
This may or may not work in different environments and you should always remember to back up your data before making changes or applying any solution regardless of source.