I believe there is advanced technology in the world.
Why?
I think its possible that at one time humans were more advanced than they are today.
Possible, maybe, but there's no reason to think it happened and reasons to think it didn't happen.
If there was a human civilisation with technology more advanced than today's technology, why is there absolutely no sign that it ever existed?
How did this hypothetical high-technology civilisation completely skip the use of metal? The relative abundance of relatively easily obtained metal ore in the bronze and iron ages makes it clear that it hadn't been used before.
The only way it could work would be with a very small and very isolated civilisation on an island that had all the necessary resources in adequate quantities and which no longer exists. Which is possible, but there's no reason to believe it.
Whether that is because some disaster happened long ago and we haven't caught back up to where we was, or technology that as been invented is being actively held back from us.
If technology was being actively held back from us (how? by whom?), then technology wouldn't have been more advanced in the past. For that to happen, the technology would have to have been in use in the past, then completely suppressed and all knowledge of its existence successfully erased somehow.
I think its possible today to build a UFO.
It's easy to build a flying object that someone else can't identify. If I build a drone with a bright light on it, fly it around and someone sees it who doesn't identify it as a drone (because the bright light obscures the drone), then that drone is a UFO to that person.
But the problem is its probably using infinite energy from a natural source of fuel, so from a financial point of view it can't be publically acknowledged.
Even if the magic you claim probably exists actually did exist, it still wouldn't make any sense to suppress it. Many countries are dependent for energy on other countries and that's something any sane government would avoid if they could do so easily. There is something else that would make suppression of this magic an appealing idea - if it could be easily weaponised. You should have gone with that as a reason.
But more importantly, where is your evidence for this magic energy source?
We know the Nazi's were attempting to build a UFO as there are documents, and even pictures of it. So its logical that the US after gaining most of the nazi's scientists would also have come across that data and looked to attempt to build it imho.
Every military is always trying to build a UFO - a flying object that other people can't identify has obvious military uses. Every experimental aeroplane was a UFO for a while, until it was identified.
If by "UFO" you mean "flying saucer" (as I think you do), well, a number of countries have tried to build flying saucers...and stopped doing so because they're crap. It's a bad shape for an atmospheric craft. It might be useful in a spaceship designed to remain in space (you could, for example, easily rotate it to create pseudogravity), but it makes no sense for an atmospheric craft or even for a spaceship designed to also work well as an atmospheric craft.