Traditionally I’m told Syrniki are best enjoyed with Smetana which is a Russian dairy product somewhere in between Crème Fraîche and Sour Cream. We can’t always find that so the best substitute would be Crème Fraîche as sour cream would I think would be to 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.