In cases where the opening date is mandatory (open award procedure), the tool asks for a public opening date . You cannot post because there is no public date , since even though you have set up the opening dates in eLicita you have not indicated that these are published on the Platform .
In these cases what you have to do is access eLicita and report, at least, a public opening date for each of the lots check the field "Publish opening dates on the platform".
