I'm seeing that as well. Looks like Broadcom has changed something or has forgotten to update an expired security certificate.
You're running the latest Fusion version 13.6.3, so there's no need to look for an update.
As a workaround you should be able to download the updated version installer disk image from the Broadcom software download site. Mount the disk image, and then run the installer found on it. That will update your Fusion installation to the latest.
As a side note, you do not have to enter any license keys for Fusion 13.6.2 and later. Because Fusion Pro is now available for free for all use cases, license keys are not required and are no longer prompted for when installing Fusion as of 13.6.2. If you've got a Fusion Player license and are running 13..5.2 or later, shut down Fusion and remove the (no longer required) licensing files found at /Library/Preferences/VMware Fusion/license-fusion-13* . The next time you fire up Fusion, you'll get Fusion Pro functionality.