I’m not sure exactly what the problem could be but when we had users getting this error on the raspi, we had them:
Insert disk into another computer
Use the disk utility to wipe all partitions
Create a new partition and format it with exfat (or whatever fs)
replace it into the server you’re trying to install StartOS on
Then try again. The installer doesn’t wipe partitions so if it doesn’t exactly like the partitions you’ve already setup, errors will occur.