I ran into this issue when I manually copied the contents of my Laravel project (say sites/oldname) into a new directory on my Mac (say, sites/newname). Since I was manually dragging and droppping, it didn't grab the hidden files, namely, '.env'. When I looked more closely at sites/oldname I saw .editorconfig, .env, .env.example, .gitatrributes, .styleci.yml, etc.
The error went away once I copied the hidden files to the new directory.
So, "No Application Encryption Key Has Been Specified" is Laravel speak for "your .env file is missing."