If you are positively sure that the locking tab is not in the LOCK position, and the camera continues to tell you the card is locked, the camera's sensor switch might be stuck.
Sometimes, the locking tab pops out of its notch and goes missing. If you see the notch where the locking tab should be, but the locking tab itself is absent, you can tape over the notch with a tiny piece of thin adhesive tape. Be careful, if this is not done cleanly or the tape is too thick, the SD card might get stuck in the slot and you'll have difficulty getting it to eject.
Inserting and releasing the card a few times, thereby jogging the sensor switch, may clear the problem.
As a last resort, you can give the SD card slot a blast of compressed air (or blow in it yourself) from a can. Be careful because the "air" from the can might cool down the card slot to the point of causing condensation, so wait a while before turning the camera on again.