I have a 10 year old canon power shot s315 (or 1s. not sure) just bought a memory card that is not working. Keeps saying, "memory card error". what is wrong?

Memory card (size) used is probably exceeding camera capability.

Try to format it sometimes it works

I Googled the camera, and came up with this :
"Maximum quality JPEG files are around 3MB each, about average for a 6MP camera, and a freshly formatted 1GB SD card is enough for approximately 361 shots, or seven minutes 49 seconds of video shooting. It's worth noting however that the S3 does not support the new SDHC memory card format.
Read more at http://www.trustedreviews.com/Canon-PowerShot-S3-IS-review#Q3AVAtXS9kvWluG0.99".

So it looks as if you are limited to an SD card.

Expanding on 1st answer, your camera might not accept today's higher capacity memory cards.

Check the specs online and get a small card if needed.

You may need to get a used card from ebay, for example, as many smaller size cards are out of production. They would probably be quite inexpensive if you can find them.

Double check to make sure the memory card you have doesnt also need a micro sd installed in it