Canada

Unfortunately, due to the FPTP system, forming a coalition between the Liberals and the NDP is just not feasible because the Liberals don't want to cooperate unless they absolutely have to, which is rare because they are de facto in power about 50% of the time (usually a majority government, so there's no reason to cooperate).

The remaining "third party" options are either a) ideologically dissimilar to the NDP and would be unlikely to form a coalition with them, or b) have such a low chance of forming government that a coalition with them would not be politically advantageous.

For example, forming a coalition with the Greens or Bloc Quebecois would likely lead to a lot of concessions on the environment and on Francophone language rights that are simply not popular with NDP leadership, who are overwhelmingly swinging centrist (who knows why, really, it's extremely weird to see them campaign on worker protections one day while advocating for corporations the next).

The closest they've gotten to a coalition recently is a "supply and confidence agreement" with the minority Liberal government, which turned out to be pretty toothless and ended when the Liberals just... didn't really do anything the NDP wanted, lol.

The current government was kind of a coalition. The NDP agreed to block calls for an early election if the Liberals expanded healthcare to include dental. The NDP thought this would score some political points but the move was seen more as propping up an unpopular government and they caught a lot of flak over it. They have since announced an end to their partnership, called for Trudeau to resign, and will be calling for an early election.