The typical software tutorial includes step-by-step instructions for installing developer tools, editing files and code, and running commands. When these software tutorials are not executable, either due to missing instructions, ambiguous steps, or simply broken commands, their value is diminished. Non-executable tutorials impact developers in several ways, including frustrating learning experiences, and limiting usability of developer tools. To understand to what extent software tutorials are executable and why they may fail we conduct an empirical study on over 600 tutorials, including nearly 15,000 code blocks. We find a naive execution strategy achieves an overall executability rate of only 26%. Even a human-annotation-based execution strategy while doubling executability still yields no tutorial that can successfully execute all steps. We identify several common executability barriers, ranging from potentially innocuous causes, such as interactive prompts requiring human responses, to insidious errors, such as missing steps and inaccessible resources. We validate our findings with major stakeholders in technical documentation and discuss possible strategies for improving software tutorials, such as providing accessible alternatives for tutorial takers, and investing in automated tutorial testing to ensure continuous quality of software tutorials.