The message is defined here, and the check is done here.
The conditionType is null when you don’t specify a attribute of ‘Condition Type’ in your condition occurrence. This field is described here. Most domain tables will have the ‘{domain}_type_concept_id’, which the checker referenced above will look for null.
Documentation on the Condition Occurrence table is here.
I’m not sure how concerned you should be about this warning, at one point it was a CDM suggestion to specify the condition type in your criteria so that you can specify which ‘provenance’ the source data came from, but in my experience, this isn’t really necessary to specify (shouldn’t all data base treated equal?)
If you want to get rid of this warning, you have to check each of your criteria and ensure you have the ‘type’ attribute specified and filled with the concepts representing the provenance you are interested in. Again, I’m not sure this is really a necessary step, but that’s what the warning message is displaying.
Hope that helps.