Q: i can't get a download button for el captain is there a reason?

Since you don't specify which iMac you have, check out Apple's system requirements to see if yours is on the compatible list.

If it is, have you installed the 10.6.8 compatibility update for the App Store?

Does your iMac meet these El Capitan hardware specifications? If so, are there any outstanding firmware updates, or pending OS X 10.6.8 updates, that you have not applied to it? If you have got this far, how much installed RAM and free storage ( Menu : About This Mac) remain available on your iMac?

If all is well in the first paragraph, did you sign into your OS X App Store with the Apple ID associated with your iMac? If you didn't then you will be blocked from downloading El Capitan and other applications.