This is a common issue which occurs if you try to launch a 32 bit app on 64 bit Linux.
Fortunately it's easily fixed.
First open a terminal and install 32 bit support:
sudo dpkg --add-architecture i386
sudo apt-get update
Then add the 32 bit versions of the libraries used by Steam:
sudo apt-get install libgl1-mesa-dri:i386 libgl1-mesa-glx:i386 libc6-i386
Then, hopefully, you should be good to go on launching Steam.