I though this question could use a tag about intonation: Guitar isn't playing in tune
The problem is that "intonation" can be used differently in other musical contexts, such as in singing. So we'd need guitar-intonation or something like that. I'm also not sure whether tuning is sufficient. Obviously tuning in general is a superset of tuning for intonation, but I'm not sure if it would be better to have a specific tag as well.
[tuning], [repair], and [maintenance] are a bit too general for this purpose. There are many questions already about tuning the guitar and dealing with tuning problems, and they're not the same thing as setting the intonation on a guitar (even though badly set intonation can cause tuning problems). Intonation on guitars and bass guitars is a bit of a black art, and one most players will encounter eventually. However, the [intonation] tag clearly isn't going to work now.
I recommend using [fretboard-intonation] as the tag for this concept: It's clear, and would apply to guitars, bass guitars, mandolins, ukeleles; anything with frets and a bridge.
Although [intonation] in itself is unclear, the combination of [guitar] and [intonation] isn't. But a guitarist is unlikely to search for [guitar-intonation]. So I think [intonation] can be used with no problems here.
I'd probably have used the repair or maintenance tags (because to me, it seems like that poster's instrument is broken our otherwise not in proper working order), but tuning is a good choice.
