I've found that sometimes (especially in Canada) the thread is metric, but the nut itself may be 15 or 16 mm.
It's the metric thread that counts.
If you can thread the nut on to your bolt, that's all that counts.
You just use an SAE socket or wrench to tighten it.
Make sure the nut threads on easily.
An SAE threaded nut will thread on a few rotations before it binds.
It feels right because you get it on a ways, but it's not the right thread.
Make sure it's metric threading.
The size of the nut doesn't matter.
View attachment 303306
View attachment 303307
An SAE nut is very close to the metric equivalent.
If the nut is too big, chances are that the thread is not metric, but not necessarily.
Just make sure that the nut threads on all the way easily.
I know Home Depot has a metric section and an SAE section. (At least in Canada. )