"There is nobody in the library."
Translation:Nu este nimeni în bibliotecă.
When using prepositions you usually do not use articles. There are exceptions, but this is very consistent with prepositions referring to a certain place/direction you're going: la, în, către, spre etc.
Except if adding an adjective to the prep+noun, in which case an article must be used:
"în biblioteca mare" (in the big library)