Check this KB article: Converting a Linux operating system fails with the error: Unable to create virtual machine (1023173)
Not sure if apply to version 5.5, but 5.5 release notes says the same:
Source volumes on top of volume managers other than LVM are not recognized during conversion of powered on Linux machines
Converter Standalone recognizes only managed source volumes that run on the LVM volume manager. Other volume managers, including but not limited to Veritas Volume Manager (VxVM), are not recognized.