Using an iMac 27 inch - Late 2012. I deleted my bootcamp partition and lost 300gb. I didn't let it uninstall properly, rushed and closed the window and thats where all the errors and lost space problems stemmed from. Can anybody help me, I do not want to format my hard drive and lose everything because I can't back it up.
Your APFS container is not the maximum size it can be.
You can expand the container to all the available free space using
diskutil apfs resizeContainer disk0s2 0