See this thread Need help entering serial number after downloading solid works.
Are you using the Download Only method?
Does your school have a Student IT Help Desk that can help you download the install files?
What about your instructor?
I simply check out my USB to students.
Once and done!
I am doing exactly what is pictured in that thread but the download process won't start due to the error message I originally described. I don't know how to download the install files without the installation manager. If I can, I would do that and then try to install Solidworks and then activate it using the serial number Solidworks emailed to me.