I have a serial number but I don't know where to enter it to keep my subscription running
The subscription model requires that you install Lr from the CC Desktop App: this doesn't need a serial number.
The stand-alone does - and it's likely that you're running the stand-alone version of Lr.
You need to uninstall this version, and install again from the Desktop app (you won't lose anything by doing so) at which point you'll be good.
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