Hello Juan,
Sorry for delay in response.
For the ML activation, you need to go to Spro-> Controllling-> Product cost Controlling->Actual Costing /Material Ledger->Activate Valuation area for material ledger.
You can do it at plant or at company code level.
You also should check the pre migration check as activation of ML is now a part of pre migration in S/4 HANA Finance.
Check my blog for the nodes,
S/4 Finance- Blog series-6- 1511 changes in details
One last point would be to activate it at Company code or at plant level.
Please check and decide,
Regards
Rishab