Hello Michalis Nearchou,
Ensure that the "Make to Order" (MTO) and "Manufacture" options are enabled for the storable product, and that the components are configured as storable products.
I have demonstrated the process of creating child manufacturing orders (MOs) in this link.
https://www.awesomescreenshot.com/video/36860828?key=ea36d5d9789e1a94d5cc73d1d8620617
If you're still unable to create the child MO, check if any installed custom add-ons might be restricting the process.
Hope this Helps,
If you need any help in customization feel free to contact us.
Thanks & Regards,
Email: odoo@aktivsoftware.com
Skype: kalpeshmaheshwari