See this thread... Toad 13.1.1.5 with Oracle 19.0.0.0.0 issue
Is your Oracle compatible parameter set to "12.2.0." ? If so follow the resolution steps in the other thread. There is a regular expression that validates the Oracle release version of both your database and its compatible setting and a release string with a trailing period like that is not handled. I will look at the regex and see how to enable it to be more flexible while remaining useful for its intended purpose.