My GoPro says there is no SD card, but I have one in and it fits.?

SD cards only fit one way. Possibly the card it to large of memory for it to work or too high a speed. Check with another card.

It's not unusual for a SD card to be bad, even if purchased new. Try another card.

Have you tried another card to see if it works, if even other cards don't work there may be a problem with the camera, also try deleting/moving everything possible off the sd card to see if there is something on there conflicting with the gopro.

Format the card in the computer, install to the camera then try again. If it still does not work, try another SD card. No sense arguing with the camera. You won't win.

Check if the slide in sd card is in unlock position

Be sure it match the Go Pro's spec as I have found some that don't work. It is true, Go Pro do not like not having a card as they have no memory themselves. Not being the computer geeky, I just go by what works.

also be sure it's in right. I know its hard to get in wrong but I just never know.

I have accidentally put the card in upside down... take the card out, turn it over and re-insert.