Backflush with HU in Repetitive manufacturing

Hello

i'am trying to do a backflush with HU in repetitive Manufacturing with tcode MFHU

i would like to do a GI of components based on Components HU and post the GR on planned order

to resume i have product A_ finish good with component B; both of them managed with HU

i have a planned order on product A , i need to do a GI with HU of component B and a GR with HU of component A

as of today the MFHU is doing only a GR with HU on A. when i'am trying to do a GI with HU of compoennt B i have

"Invalid combination of material and planned order" error message.

can you please tell me what i'ma doing wrong: which should be the process end to end to be abmle to run such scenario

thanks regards

Lena