Troubles with co-registering brains

I tried several times to overlap 2 scans with the Elastix as well as the General Registration (brains) without success. Elastix would complete the registration, but it was far off and the general registration tool would say “error”. I have used both of these successfully in the past, so I don’t feel like I am doing anything differently. I am using version 5.8.0. Could someone help with this? Thank you.

hi @BDhoth im a beginner with 3d slicer i want to co register PET and MRI can u guide me if u have already done it before??