"Getting Started" and stuck. There is no "wall", and the 1x4 seems-to-be-wall cannot be placed anywhere.
You can place it - rotate it so that it's vertical and place it at the bottom of the middle column.
The idea was to teach you that you can place tetronimoes over the lava blocks.
I can do a few things to make that clearer though:
1. Add a hint that pops up
2. Make the hint color on the blocks even clearer when you're in a valid or invalid spot (the color is a bit subtle atm).
Thanks for taking the time to give it a go - sorry you got stuck / the tutorial wasn't clearer.