There's a big header at the top of that section titled "If you are running ubuntu core." Judging from your prompt (user0001@laptop) I'm guessing that's actually not the case for you.
The classic snap is only to obtain a "classic" developer environment on Ubuntu Core, which is completely snap-based (e.g. no apt, no debs, etc.). In your case, I'm guessing you're already running on a "classic" (i.e. not snap-based) distribution, in which case you don't need the classic snap, and can simply move on to the next step.
In fact, the beginning of that section says pretty much just that:
Ubuntu Core is not a traditional Linux distribution: there is no "apt", the system is read-only. How can you develop and build snap in such an environment then? We have a special snap for this!
Enter the "classic" snap. This snap will get us into a chroot, where we have a traditional Ubuntu distribution. We can there install more packages, change files projects and run snapcraft to build our snap. The home directory is shared between the container and ubuntu core. We can thus build a snap, then, outside of the classic snap, install and test it.