Hi @Afrosheen,
This is related to a recent expiration of a root certificate previously used by Let's Encrypt, including by the Let's Encrypt service itself.
This error indicates that the QNAP OS would need to be updated to one that includes the ISRG Root X1 root certificate authority.
This root certificate was created about five years ago and became part of most popular operating systems released after that time. But it's likely from this error that your QNAP software is based on a system that predates this, and therefore would need to be updated.
I would suggest checking for updates via whatever kind of software update mechanism QNAP provides; if you can't find any, I would suggest looking for relevant discussions at the QNAP forum
or, if there are none, posting your question over there.
Good luck!