The only thing different between your print panel and mine is that the panel is being presented to you by your HP OfficeJet 250 printer driver, and I have no HP printers here. If the driver for that printer is out of date for Big Sur, that may be part or all of your issue. Your particular printer is supported by Apple's AirPrint which is built into the operating system, so you don't need to visit HP for drivers.
If you are not already using AirPrint to support this printer, first ensure that the printer is powered on and networked either wired, or wireless. Then visit System Preferences : Printers & Scanners panel and right-click on the image of your OfficeJet printer. Choose Reset Print System… and it will remove, and then present a dialog to re-add your printer. Instead of formal HP drivers, choose AirPrint instead, and re-add the device. Right-click on the printer icon again, and make it the default printer. Print something to ensure it is working. Then repeat your Preview exercise to see if the PDF : Save as PDF sub-menu is now active.
As an annoying workaround, you could also choose Save as Postscript, and once that document is saved (.ps), you double-click it in the Finder to have Preview automatically convert it into a PDF and open it — which you then save separately.