This may be known to others, but since it’s been a few years since the last major vSphere came out, I’m sure some others will run into this.
If you buy NEW licenses, they will come as vSphere 6, which is incompatible with a vCenter 5.
If you try to add the license you will see this error:
In order to fix the issue you need to downgrade the vSphere license
Go to the license portal, choose “I want to downgrade” and select the vSphere 6 license
Select the appropriate number cpus to change (in this case 4) and choose check the box
Pretty straightforward thing to do, just something that you may overlook as VMware moves on with new licenses while you are holding off on updating to the latest hotness.