Both the plural (visas) or the singular with an article (a visa) should be accepted because with N class words (such as viza) there is no distinction between singular and plural
Why is it na visa? Why wouldnt you just say wasipokuwa visa
"Visa" should be plural here.
There is such a thing as group visas; all that is needed is an article.
The English is more natural if its says "If the don't have a visa (or visas) they will not come to Tanzania"