Translation:The little mermaid is not in time for kissing the prince.
In this sentence, "når" is a verb in the present tense and it means "make it" (in time). It's a convenient word that's missing in English. "I can't make it" (referring to a time issue) translates as "jeg kan ikke nå det". "Nå" can also mean "reach". When it's used in reference to a temporal deadline, you can see it as a metaphor, where space is substituted for time. A hypothetical event happening at a particular point or interval in time becomes a point or area in space which is either within your reach or out of it.
It's a good suggestion for an English translation of the sentence. However, in the Danish sentence, it's specified that the reason why the little mermaid doesn't manage to kiss the prince is that she's too late for it. It is implied that she would have managed it if she had had more time. It's not because she doesn't know how to kiss or because the prince doesn't want to kiss her, for example. This is not clear from the English sentence. So this is one of the cases where Danish has the upper hand in comparison to English.
That would be a perfectly sound English sentence but it would erase the temporal aspect of the original sentence. The reader would no longer know that the reason she failed to kiss the prince was related to time: Either she was too busy, too late, or she was cut off and never made it to the point in time when she would have kissed him. It's a hard sentence to translate.