Trying to decimate, but I'm getting an error

I am trying to use the decimator tool, but it wont apply to the mesh because of some error that says Modifier error: Non-manifold mesh as i. Then when I try to apply it, a message pops up that says Modifier is disabled or returned error, skipping apply.

Does anyone know what is going on here?

Thanks.

It might be that your mesh has a hole in it somewhere, check for any gaps.

In edit mode, Select>>Non-Manifold [Ctrl+Alt+Shift+M] will find the gaps for you. Let the computer do the work.