Initially, I had dual-booted Ubuntu on my Mac. Several months later, I decided I no longer needed Ubuntu on my Mac so I decided to get rid of it. I attempted to this by using Disk Utility on macOS and erased the partition. It didn't work, only the swap partition was erased.
Afterwards, I've then accessed the Ubuntu version on my Mac, afterward, I found myself no longer able to access the Mac OS.
I found out that my mac partition had the same FFFFFF-FFFF-FFFF-FFFFFFF problem as this post (Partition type suddenly FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF, drive unmountable)
I followed the instructions of the selected answer but now I can boot into neither of the OS (Before at least Linux was still bootable)
The current state of the machine is as the picture below:
My mac is a 2013 MacBook with 252 GB of SSD drive. It was running OS Catilina (the latest).
I'd really like to obtain my macOS back along with the data/files there.