Can mermaids go on land without dying?

Some folklore says that they can't, but most recent stories show them sitting in open air on rocks near the sea, indicating that they're either mammals or amphibians. This would mean that they could do so without issue. So you can rest, as there is plenty of imagery that supports that they can.

There may also be an issue with them being away from the water for too long, since there is a good chance that they have skin similar to dolphins or frogs, and so drying out might be painful or perhaps might kill them.

But not everyone agrees, as most mermaid folklore is made up stories (where it's not an exaggeration). Some old stories keep them to the water like sharks or dolphins, while others have them able to turn into a human with magic, and still others say they can become human at will.

You might try to write your own ideas about how this might work, and look around the internet for pictures of mermaids on rocks.