Let's say you fit an ARMA-GARCH model to financial data and find that the standardised residuals are non-Gaussian through the Kolmogorov-Smirnov test. These residuals have mean -0.002 and standard deviation 0.997.
From this reference one cannot use the Ljung-Box test of autocorrelation in my residuals because the data needs to be Gaussian and mean-zero (it is mean zero). What test can be used then to check for remaining autocorrelation and heteroskedasticity?