I have followed a tutorial and it does not work
There are multiple reasons that an operating system based on a tutorial may not work.
You forgot a step
This is most common if you copied and pasted the code without reading the explanation of what that code does. Re-reading the tutorial may help solve the problem.
Your tools are too old or too new
Features are added and removed from devtools all the time. If the tutorial is outdated it may be written for users of an outdated tool like TurboC or DOS. If it is too new then you may have be using tools which don't have features that the tutorial uses. If the tutorial is too old then you should find a newer tutorial or write it yourself without using a tutorial. If the tutorial is too new you should update your tools to the latest version and check the "Prerequisites" section in the tutorial if there is one.
There is always the chance that the tutorial might have bugs in it. If the tutorial has not been updated ever, the chance for bugs is higher than if the tutorial is updated regularly. If the tutorial is on the OSDev Wiki, you should check the Discussion tab and also check if the article is disputed. However, tutorial bugs are less common than you think so you should make sure that you did not screw up before you blame the tutorial author.