When should I get an insurance quote on a house I am buying?
During the inspection period, not after. Insurability varies far more than buyers expect, and a wildfire-exposed property or one with an old roof can be expensive or difficult. Finding out while you still have an inspection contingency is the whole point of having one.
What should I ask the seller?
The ages of the roof, electrical panel, plumbing and water heater, and whether there have been any claims. Those four dates decide most of what a carrier charges, and the claims history follows the address to you whether you ask or not.
Can a house be uninsurable?
Rarely outright, and frequently expensive or limited to a narrower market, particularly with wildfire exposure or very old systems. That is a negotiating fact as much as an insurance one, and it is better known before you commit.