It would appear the json data type is an unrecognised column type when using mysql, except mysql 5.7+ does has a native json data type. I'm going to assume that the root of the problem lies in this file: https://github.com/phalcon/cphalcon/blob/master/phalcon/db/dialect/mysql.zep#L197
The line shows the error that is thrown. I also assume it's just a simple case of adding a case statement to handle json, so I've forked and submitted a pr for version 3.2 - https://github.com/phalcon/cphalcon/pull/12984
Also, I would have thought that instead of showing this error, the fallback would have been to map to a text column instead?