Hola Emanuel
In Peninsular Spanish we use preposition "por" to say "in the morning", "in the afternoon", and "in the evening". In some parts of Northern Spain preposition "a" is sometimes used too, but this is not the general use.
You may have heard "en" but that would be in some parts of Latin America, not in Spain.
Kwiziq is focused on Peninsular Spanish at the moment, although we plan to have a Latin American version too at some point in the future.
Un saludo
Inma
