The format of the apriori file has two columns. The left column has the class IDs. The right column contains the a priori probabilities for the respective classes as values between 0 and 1. The values of the a priori probabilities cannot total more than 1.
Ensure that the specified apriori file exists and is in the proper format. Check the Class Probability or Maximum Likelihood Classification documentation for further details on the formatting of the apriori file.