I think you need to check the normality

The diagnosis checks if the hypotheses
made on the residuals (actual prices minus fitted prices) are true. Residuals must satisfy the requirements
of a white noise process: zero mean, constant variance,
uncorrelated process and normal distribution. These requirements
can be checked by taking tests for randomness, such as
the one based on the Ljung-Box statistic, and observing plots,
such as the autocorrelation and partial autocorrelation plots.
If the hypotheses on the residuals are validated by tests and
plots,