VMware vSAN 8.x
VMware vCenter 8.x
The "VMware vSAN 8 for Foundation" license is supported to be used on vCenter version 8.0U3 and above.
This can also be validated by looking at the Broadcom support portal > License > License description - it states that "VMware vSAN 8 for Foundation (Supports vCenter Server 8.0U3 and above)":
If it is attempted to add this license on a lower version of vCenter, the license key validation check will fail stating that the key is invalid.
To resolve this issue:
Upgrade the vCenter to version 8.0U3 or above.
Also ensure that the ESXi hosts are of version 8.0U3 or above.
Post this it will be possible to add the "VMware vSAN 8 for Foundation" license key to the vCenter and also apply it to the vSAN cluster.
For more information about the VVF and VCF licensing refer VMware Cloud Foundation and vSphere Foundation Solution License Key.