Smetana vs Creme Fraiche vs Sour Cream
Traditionally Syrniki are best enjoyed with Smetana which is a Russian dairy product, somewhere in between Crème Fraîche and Sour Cream, called Smetana. We can't always find that so the best substitute would be Crème Frich, as sour cream would would be too acidic.
For my friend and family reading this in the U.S. I can't remember exactly what dairy products you have. I know you have Sour Cream. Maybe there is a less acidic version in stores as well.