I am getting an message "memory card error" on a Canon G6 on 2 different cards

It's a lost cause. The A540 works with SD cards. While SDHC cards will fit into the slot, they are electronically incompatible and will not work in devices not designed for them. You need to stay with SD cards, 2GB and smaller.

Welcome to Fixya
Err 99 could mean several different things, such as... Memory Card, Lens or Lens contacts....

Here's what you can try:
Remove the memory card an enable "shooting with out memory card" and see if you still get the error. If it works your memory card is bad. Try another lens if you can. Clean the lens contacts with a pencil eraser (be careful not to get eraser dust in the camera body or in the lens). Worst case scenario is, Err 99 could be related to the shutter failing which would mean a trip to Canon.
Thanks

If you need further help, reach me via phone at https://www.6ya.com/expert/elixirjose_00375ea24bd8141b

Welcome to Fixya
Err 99 could mean several different things, such as... Memory Card, Lens or Lens contacts....

Here's what you can try:
Remove the memory card an enable "shooting with out memory card" and see if you still get the error. If it works your memory card is bad. Try another lens if you can. Clean the lens contacts with a pencil eraser (be careful not to get eraser dust in the camera body or in the lens). Worst case scenario is, Err 99 could be related to the shutter failing which would mean a trip to Canon.
Thank you

If you need further help, reach me via phone at https://www.6ya.com/expert/elixirjose_00375ea24bd8141b

Err 99 could mean several different things, such as... Memory Card, Lens or Lens contacts....
Here's what you can try:
Remove the memory card an enable "shooting with out memory card" and see if you still get the error. If it works your memory card is bad. Try another lens if you can. Clean the lens contacts with a pencil eraser (be careful not to get eraser dust in the camera body or in the lens). Worst case scenario is, Err 99 could be related to the shutter failing which would mean a trip to Canon.
Thank you for using Fixya!

If you need further help, reach me via phone at https://www.6ya.com/expert/elixirjose_00375ea24bd8141b

From page 145 of the A520 user manual, it appears that the largest SD memory card that this camera is able to use is a 512MB SD memory card. The manual is dated "2004", so this size limitation is not unusual for a camera of this era.
If you've purchased anything larger, there is a good chance that it won't work with this model camera.

Having gone over a month with no response, I assume it was just a matter of SD versus SDHC cards.

Cards are coming in different formats and it might be possible that you brought a card which is not suitable or functional with your camera. The other reason could be that we are having options to lock our memory card in digital camera and you might have taken out your earlier card without unlocking it. The system might be broken from inside and this could be a reason that you are still getting a message that memory card is locked. It seems possible based on problem you are sharing.

try to reformat the card either from camera or pc. If reformating doesn't work, then try a different card. Some camera's are touchy when it comes to the memory cards

I get a memory card error everytime I insert a memory card. They are not locked and I have tried 4 different memory cards and nothing works. How can this problem be fixed?

Same problem here with Micro Memory MMSD/4GBHI card. What to do?