Hi PinkSin and welcome.
You cannot register your Vaio because you are using the wrong Model code.
PCG-91311L is the designation of a whole series of Vaios of which yours is just one, but for support and other conversations with Sony you will need to use the correct Product Name for the individual model.
Have a look on the screen surround for the correct model - it will probably start with something like VPCEJxxxxx.
You can also find this Model Name in Vaio software on your Vaio - try to find and open 'Vaio Control Centre' and you will find it under System Information. The Model is also shown in the BIOS.
When you find the correct Model, try to register it online - you will probably find the model starting VPCEJ but if not you may have to phone Sony Support as this model probably dates from 2011 and will be way outside any warranty arrangement. I can only see a few VPCEJ models listed there.
https://www.sony.co.uk/electronics/support/laptop-pc-vpc-series/articles/00202250
As an example, here is a link to the Sony Support website page for the VPCEJ1M1E:-
https://www.sony.co.uk/electronics/support/laptop-pc-vpc-series/vpcej1m1e?downloads-num=95#downloads
