Translation:I want to go swimming tomorrow evening.
They are utterly inconsistent with the use of 晚上 In one answer it can mean night, in another it can mean evening. You often get marked wrong for predicting one or the other. There is already one example in this lesson where the meaning is night where you would not predict it, but in this question here it is supposed to mean evening.
The example is: "My younger sister runs every night". In this case 晚上 means night. Now here with this answer it suddenly means evening and night is unacceptable. Sheesh!!!! It is so irritating, but don;t hope for change, I notice you wrote this post a year ago.
People here are wondering why " i want to swim" is not accepted when it clearly is self-answering (There is clearly a 去 in the sentence, which requires "go" to be written).
However, last exercise was without a 去 and they all went writing that 游泳 is equivalent to " went swimming" when it isn't essentially that.. very weird, people