I should mention that I had to set the gap of the differential pair wider than that of the default track clearance, for example, the gap = 0.009 mil, the default tack clearance = 0.008 mil. Otherwise, the ERC reported errors like “track too close”, even the gap = 0.008, the default track clearance = 0.008, would get the ERC errors.
I was sure I have seen somewhere the possibility to set differential pair.
I spend only 10 minutes and I found it: File - Board Setup - Design Rules - Net Classes.
I was here several times until I noticed that last parameters for each Class are dPair…