At a guess...
You are trying to reserve more CPU for your VM than is available on your host.
An apparently common mistake is to forget that the host reserves some CPU for its own exclusive use. Approximately 10%.
Another is to overlook that a 2.4GHz CPU may not be exactly 2.4GHz.
So for example my test host has an Intel XEON E5-2609, which is a 2.4GHz processor.
If I look at Host > Configure > Hardware > Processors, this tells me that the processor speed is actually 2.394GHz.
So rather than 19,200MHz CPU resource (8 x 2400, 8 being the number of logical processors on the host), my host actually has 19,152MHz total CPU resource.
Now looking at Host > Configure > System > System Resource Reservation, I see the reserved CPU is 239MHz per CPU.
So the most I can reserve for VMs is (2394 - 239) * 8 = 17,240MHz.
