Hi Ismael,
If you check the F1 Help for Mixed Storage in the storage type control (transaction OMM3), you will see:
" A " Several Storage units with the same material per bin
Several different materials or batches of a material may occupy a storage bin at the same time. However, the individual storage units must contain the same material and/or batch number.
Kindly view the other options for Mixed Storage to suit your requirement. If you require only one specific material & batch per bin, leave Mixed Storage field blank.
You may also find this documentation helpful:
Strategy P: Storage Unit Type - Warehouse Management System (WMS) - SAP Library
Regards,
Neil