Why is VAT period Nov 19 - Feb 20 showing a liability when no sales for period?

It could be any number of things - without access to your QB file, or clairvoyance, no-one is going to be able to give you an answer ;-)

There are several VAT reports in the Reports Section - try drilling into them or looking in the VAT account for the errant amount?

You should be able to identify where the liability is coming from.