From the manual here: https://onlinemanual.nikonimglib.com/z50/en/80_troubleshooting_03.html

Message: Full (flashes)

Memory insufficient to record further pictures at current settings, or camera has run out of file or folder numbers. Reduce quality or size. Delete pictures after copying important images to computer or other device. Insert new memory card.

So either your memory card is full (deleting the images on the pc might not free memory on the card) or you have used all the index numbers that the camera can write, so there are no file names left or there is something broken in the camera that needs maintenance. My gut feeling would be the first thing.

Just a note in case: If you continually store all your images on the sd card, please absolutely stop doing this. SD cards are relatively robust, but they are not a good long term storage. You risk losing all your images if the card fails. And almost all photographers that I know had at least one failing card in their life.

Get your images from the card, format your card in the camera with the camera function and you should be good to go. If that does not work, try out another memory card that you also format in camera. It also might be a faulty card.