Flyway expects three forms of instruction
This exception most commonly occurs due to poor syntax or due to nuances of the platform in use, the syntax not being handled in the expected matter.
As such it is easier to explain what a flag is, to allow user to self correct. Unlike a parameter which requires an argument to be supplied (including an implicit default) a flag operates in isolation and doesn't take an argument.
In most instances, a syntactical error, character escape etc causes a parameter statement and parameter value to become separated, causing flyway to treat the parameter as a flag, which is then deemed invalid.
The faulty value will returned in addition to the invalid flag, so it should be self evident where the error occurs, once you know what to look for.