How are software coupling and portability are related?

The concept of coupling embodies the reliability and dependability (strength) among all relationships between functional units.
The concept of software portability is the usability of same components/software in different environments.
For optimal software, it is generally the case where coupling has to be minimized and portability has to be maximized (for sustainability and effectiveness in the long run). Having said that, strong software that contains low levels of coupling could easily and efficiently is portable. Similarly, if the software is portable it is probably due to the low levels of coupling that it has.
Think of these as LEGO pieces. If you want a working building, then all those nooks and crannies should not be visible (implying some coupling). That means that they should be stuck together pretty tightly (coupling). At the same time, if you want to remove a part of your structure like a room or a floor, then the tightness of your pieces (coupling) shouldn't be as strong. Ultimately, your structures portability is only dependent on the tightness of the pieces.