Sometimes it's right to close an answered question. But that particular closure was wrong. The sub-reason of off-topic that was selected was:
This describes a problem that can't be reproduced that seemingly went away on its own or was only relevant to a very specific period of time. It's off-topic as it's unlikely to help future readers.
But this problem didn't just go away by itself, and would occur again for the OP or anyone in their situation when installing on a machine where the "install alongside" option isn't available.
Instead, what happened was that the OP solved the problem and told us how. And the way it was solved appears to be the way that is described in the answer the OP accepted.
The question is related to How do I install Ubuntu beside Windows 7 using “Something Else”? and How to get Ubuntu installer to sense Windows 8.1 as an OS, and it may be a duplicate of one of them. If so, it could be merged into one of them.
We don't actually have to reopen and dupe the question to get the answer merged. My understanding is that a moderator can merge answers from any closed question, even if it wasn't closed as a duplicate.
Unless we want to go that route, since this question is now getting delete votes (some people don't realize that, for the most part, deletion is for garbage), we may as well reopen it. It's an answered, on-topic question that will likely help others, and if people think it's a duplicate of some other question, they/we can vote to dupe it.
Before the recent phenomenon of widely targeted delete votes on non-duped closed questions irrespective of whether or not the question or answers were valuable, I'd likely not suggest reopening a question that would likely be closed again soon after. But in fact the off-topic closure was wrong, reopening it is not wrong, and in the present context reopening is likely the best thing to do to prevent deletion of useful content.
Note that we should not reopen questions that are actually and truly off-topic and cannot be reasonably interpreted as on-topic. But this particular question was never off-topic.