Well then is invalid !
there is no other reasons behind it
In the vcenter where you are inserting the license the nsx manager is registered? Otherwise this error can occur even ...
Call VMware support and navigate to the license team to confirm it's a valid key and there are no issues with it.
I have had this issue also today. I tried a several license for NSX, no sucess. After we have booted the vCenter appliance and the NSX Manager all was running fine and the license was valid.