It depends on the profile that you are interested in, as in if you want to become an SAP MM consultant, then it's most recommended to have in and out knowledge of SAP's Materials Management module, its business processes, the SAP MM sub-components and a lot more. SAP MM (Materials Management) includes all the tasks within the supply chain like Purchasing, consumption-based planning, vendor evaluation, and invoice verification. This also covers inventory, production planning, and warehouse management and a lot more.
If you want to learn SAP MM from scratch to advance, then here's the right video for you:
If you are more into reading, then do check out this material provided by Intellipaat:
It also depends on your educational background and the previous experience in a particular domain. So, my personal suggestion would be to learn thoroughly the specific module of SAP on which you are targeting then go through some job portals like Indeed, Naukari ans so on and search for the relevant job openings.