The Principality of Andorra is de jure “a parliamentary co-principality with the president of France and the Catholic bishop of Urgell (Catalonia, Spain) as co-princes.” (From Wikipedia)
A visiting tourist however would likely have at least some credible reasons to doubt the sovereignty in fact. For example, the postal services are entirely supplied by Spain and France.
Also, it may be that “As co-princes of Andorra, the president of France and the bishop of Urgell maintain supreme authority in approval of all international treaties with France and Spain, as well as all those that deal with internal security, defense, Andorran territory, diplomatic representation, and judicial or penal cooperation. ” (Although I have not found official confirmation)
Which certainly suggests the sovereignty of Andorra is at best shared, and reliant on, French and Spanish sovereignty.
Since the actual situation seems better described by some other arrangement such as de facto protectorate, client state, etc., then the implication would be Andorra is only de jure sovereign.
If that is the case then the general principle seems to be that countries can possibly exist in a situation that is only de jure sovereign.