Many people are reporting their PC/Laptop is not showing in the device list on their Microsoft account online, it seems to be a server side problem that Microsoft need to fix.
Ensure you log into Windows with your Microsoft account, then open the settings app and ensure you have Location services enabled for all apps and also enable the 'Find My Device' setting, then your device will get automatically added to your Microsoft account online, that may not happen immediately, but it will happen.
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