I am using a shell weight with a cartridge which weights under 6.5 g, but I can’t balance the tone arm even when I turn the balance weight.

here is some misinterpretation in the Operating Instructions. We correct it as the below.

02 Before you start - About the cartridge
(Error) When using a cartridge between 3.5 g and 6.5 g, use the supplied shell weight.
(Correction) When your cartridge is too light to balance the tone arm, use the supplied shell weight.

02 Before you start – Assembly
(Error) When using a cartridge between 9.5 g and 13 g, attach the sub weight on the rear shaft of the tone arm.
(Correction) When your cartridge is too heavy to balance the tone arm, attach the sub weight on the rear shaft of the tone arm.

07 Additional information - Specifications
(Error) Proper cartridge weight 　　　 3.5 g to 13 g (single cartridge)
　　　 When shell weight is used　 　　 3.5 g to 6.5 g
　　　 When only balance weight is used　 6 g to 10 g
　　　 When sub weight is used　　　　　　9.5 g to 13 g

(Correction) Proper cartridge weight　　　 2.5 g to 12 g
　　　　　　　　　　　　　　　　　　　* Cartridge weight includes the weight of mounting screws and nuts.
　　　When shell weight is used　 　　　2.5 g to 5.5 g
　　　When only balance weight is used　 5 g to 9 g
　　　When sub weight is used　　　　　　8.5 g to 12 g