Mac OS X snow leopard will not install on early 2011 MacBook Pro running High Sierra

Various possible issues:

1) You have misidentified your Mac model.

2) Your Mac is incorrectly formatted, particularly if you have replaced the drive with a SSD. High Sierra can use APFS. Unlike later versions it didn't insist upon using it but if you installed it to a SSD it may have reformatted your drive to a format Snow Leopard does not recognize.

3) Make sure your computer is actually booted to the installer disc. This isn't like modern installers which can use a Recovery partition to install and you can just run an installer application directly while booted to the drive.

4) You have the wrong version of installer disc. You may need the original disc, not a retail version. Retail versions often came with an earlier version than that on a disc supplied with a computer, and Macs cannot boot to anything lower than OS with which they were supplied originally.

For example, https://everymac.com/systems/apple/macbook_pro/specs/macbook-pro-core-i5-2.3-13-early-2011-unibody-thunderbolt-specs.html

says it originally came with 10.6.6 (your photo shows 10.6.3) You could check for your exact model on that list but I suspect all early 2011 will say the same. If you want to use those retail installer discs you'll need to find a computer than can boot to them, install to a separate drive, then clone to the Mac, or boot your Mac to Target Disk Mode and install to a partition that way (unless you're wanting to wipe the entire drive).

Using that same web site, same SL version.