You've fallen foul of a Restricted Parking Zone, which being a zone covers several streets with signs at the entrances to the zone. Good councils also put in repeater signs, but they are not obligatory.
I've searched around, but can't seem to find what zone Clifford St is in, but it's very close to New Bond St, where an RPZ was introduced in 2018. AN RPZ differs from a CPZ in not requiring any yellow lines, basically, one can only park in marked bays, with no parking anywhere else.
I found this RPZ sign on Bruton St, but about 15 yards further on, there is a CPZ sign, so it looks as if Westminster have done the usual council
****-up with signs. HOwever, you can only turn right here into New Bond St
https://maps.app.goo.gl/R8H4rUfp8TwZ1KWo6So as New Bond St is one-way only, thus making you drive into CLifford St, it looks as if CLifford St is part of the RPZ.
In truth, the only way will be to look at the Traffic Order for this RPZ. As far as I can see, the post code issue won't get the PCN cancelled, but lack of signs might.
So how did you get to where you parked in Clifford St ? From the road layout and one-way arrangements ? It must surely have been via New Bond St, and thus you would have passed an RPZ sign.