BitLocker Setup requires the drive file system to be NTFS
You are right in your assumption that the Error you get is in case because you have that DELL OEM Partition with the FAT32 FS. If you want to turn on BitLocker, backup that partition, delte it, turn on BitLocker and restore the Partition if you really need it. I have not found another way yet to get around that issue.
After the deletion of the Recovery-Partition the BitLocker-Check will not fail anymore.