Hi Fernando,
Do you mean you cannot create a change version for the PO in SRM? If so, I think that version management is active for BUS2201 by default, but you can check customizing to ensure whether it is actually activated. Anyway, I think you can always try to delete or mark PO as not valid (maybe using custom user specified status if no other way) and try to create another one from corresponding SC. It is also possible to manually set again SC items as "ready for sourcing" dealing directly with items table (/h from bbp_pd).
I hope this helps
Regards
Vicente