I read that Random walk with or without drift are not covariance stationary. since mean reverting level is b0/1-b1 = b0/0. How is mean reverting level is related to covariance stationary.

If the coefficient on the slope =1, then the denominator in the mean reverting level equation b0/1-b1 is zero (1-1=0!). Anything divided by zero is “undefined”, and anything with an undefined mean reverting level cannot be covariance stationary.