I followed the instructions on this link (last answer) to delete the Core Storage Logical Volume Group via terminal while pressing Cmd+R during startup. These are the functions I used in terminal:

diskutil cs list
diskutil cs delete <Logical Volume Group UUID>

Now I cannot resize the partition via Disk Utility. The post mentioned that a red error message would appear when selecting the drive, but that didn't happen. I'm stuck!

These are screenshots from the errors in Disk Utility:

After resizing the top partition by dragging down and clicking "Apply", an error message prompts: "Couldn't unmount disk". Also happens after I repair the disk.

When restarting the computer, a grey screen with error icon appears.

Any help would be greatly appreciated!