Having a brain injury can affect people in different ways.  Many people struggle to remember simple things, such as their PIN number when paying by card.  It is possible to have an arrangement with your bank if you are experiencing such a difficulty so that when you put your card in the card reader it will ask for a signature instead.  It seems however that not all shops appear to be aware of this.

https://www.theguardian.com/money/2019/jan/20/chip-signature-card-disabled-refused-shopping-impact