How to go back to the Ruins in Undertale?

I read about sparing Muffet in Undertale (option 1), so I backtracked towards the door to the ruins. But when I tried to go back inside, it restrained me from going. Anybody have an idea as to why this happens?

If so, please tell me :/

You can't return to the ruins once you have left. The only exception is after the final battle of the true pacifist run.

Unfortunately, this means there is no way to get Spider Donuts or Cider to skip the Muffet fight without paying 9999g.

It's possible to return to the Ruins eventually, but the trigger for that scenario happens after the Muffet fight.

If you missed the stuff needed for Option 1 of the Muffet fight, you'll have to spare her the hard way.