Inability to unit test is usually either a symptom of poor system structure (e.g. components are inappropriately coupled) or an attempt to shoehorn testing into the wrong spot.

If you find yourself trying to test a piece of code and it’s an unreasonable effort, try moving up a level. The “unit” you’re testing might be the wrong granularity. If you can’t test a level up, then it’s probably that your code is bad and you don’t have units. You have a blob.