Hi @gbelectrical and welcome to the Cayenne community.
I'm confident at the moment that our certificate is OK, my first suspicion is that the time/date on your Pi where you're seeing this error may not be set properly, which can cause cert validation errors. Raspberry Pis can be funny about time -- they don't have any built-in clock battery like a proper PC in order to cut costs on the board, so time/date come from an NTP time server at boot time. This can fail on occasion leaving the system time incorrect.
Could you run the command
date on your Pi and let me know if it matches up with the real world time and date? If it doesn't, I would suggest a reboot first, then running