The Macbook Pro isn’t a device we support, though many DIY users have successfully installed StartOS on a Mac. If you search the forum you’ll find your issue discussed at least a couple of times. One I remember clearly is this one…
Having installed StartOS on a Mac myself and having similar trouble I ended up pulling out the drive, connecting it to another computer and wiping+partitioning it manually.
I am not sure if it will help.
In my case, I use my old macbookpro end 2013 uninstalled MacOS and install Linux system, i updated the BIOS version , then flash USB with start OS and deleted all Linux.
My spec: 8GB with 256SSD internal + 2TB external.