I see this has been a problem with other games, too.
Making a new account won't help, it appears the trial is blocked by computer hardware id of some sort - so if you or someone else in your household has used the trial, you can't rerun it again. This is to prevent people from just creating a new account and running the trial over and over instead of purchasing. This isn't really a game-specific thing, it's Origin.
A couple of things to try - Clear the Origin Cache, then reboot
Make sure to set *.exe for Origin and the SimCity trial "as Administrator" (right-click desktop icon, choose Properties > Compatibility > Check the "Run As Administrator" checkbox)
Right-click SimCity trial icon in Origin, choose "Repair"
There's not much anyone in the forums can do to help with this, if none of these options work, you'll need to contact an EA advisor and request help.