I installed AmiKit 1.2.0 and then AmiKit_12x-130 update. I used OSXL CD as a source of system files. When the AmiKit desktop appeared, I installed BoingBag2 in Expert mode and everything worked as expected. No error appeared.
Maybe the CD filesystem used in AmiKit is not 100% compatible with your CD drive and thus the CD is not recognized correctly. Before you start AmiKit, try to mount your CD-Drive as a Directory.
Go to "Hard drives" section of WinUAE settings, press the "Add Directory" button.
Device Name: CD4
Volume Label: AmigaOS XL
Path: point to your CD drive with the CD inserted
disable Read/Write option