I wasn't allowed to add the "Windows 10" tag to my question, which was specific to Windows 10. What is the purpose of prohibiting people from asking questions specific to Windows 10?
The reputation requirement doesn't prevent you from asking a question about Windows 10; rather, it prevents you from creating a tag windows-10. This is to avoid multiple 'wrong' tags being created; it takes a while for a new user to get the concept of tags (even experienced users get it wrong regularly); therefore, it requires 300 reputation to create a tag (on a beta site, it's 150 reputation). Because of past waves of tag creation 'abuse', it requires even more reputation on Mathematics Stack Exchange (1000) and Stack Overflow (1500); see Reputation requirements compared.
You need a minimum reputation to create a tag. On say, Super User, where windows 10 is on topic there's a preexisting tag. you can already use windows-10. On other sites, there may be less of a reason to.
Since tags are often created without a real need (and it's a little too easy to make a bogus tag), the minimum reputation level needed to create them is a nice way to ensure people who create new tags know how things work (and to keep spammers from abusing it, 'cause they're why we can't have nice things).
Assuming you were trying to ask the question on Stack Overflow, you had multiple existing tags to choose from:
No need for a new tag.
If you need to ask a Windows 10 question on a site that doesn't have that tag, you should first make sure your question is on-topic on that site. If it is, you can still ask it, but without the Windows 10 tag.