How do you sign up for summer school and you've just finished first year but want to take a second year course? It keeps saying "pre-requisite failed" but there are no requisites for the course except being in the program.

Keep trying. You may just need it to show up on your degree audit. I would email your advisor since school does start soon

What course is it that you are trying to take? Are you officially registered in the program?

You should definitely email your advisor about it. The deadline to add courses is coming up soon. If possible, i would just speak to the advisor in person since the first year advisors are usually busy at this time of the year and may take some time to reply to emails.

If the prereq is "being in the program", you don't meet the prereq because you aren't in the program yet - you aren't considered a second-year student until September. You need to talk to someone from the department that is offering the course you want to take, and they can give you a requisite waiver. An academic advisor probably can too, but as Jim said they tend to be busy at this time of year.