That is indeed a different problem — it's a mismatch of data types.
The setEnabled slot has a slightly confusing name. It could be read as "set
widget enabled" or as "set the enabled state of
widget". It actually has this second meaning and takes as input a truth value — "enable me" if true, "disable me" if false.
The calendar widget's clicked signal carries a QDate. QPushButton doesn't know what to do with a QDate so Designer won't let you connect the two.
Our standard practice in a case like this is to handle it entirely in code:
var calendar = something; // some way to get the QCalendarWidget
var button = somethingElse; // some way to get the QPushButton
if (something about pDate is acceptable)
button.enabled = true;
button.enabled = false;
// or a shorter version
button.enabled = (pDate is acceptable);
Hope this resolves the problem for you.