When facing the error M3150 The company code XX does not exist or has not been fully maintained, for example during material creation in transaction MM01, the root cause is most likely that no fiscal period has been set.
As described in the Performance Assistant, go to Maintain company codes for materials management, using either the direct link provided
Or by going in SPRO, in Logistics – General > Material Master > Basic Settings > Maintain Company Codes for Materials Management
Here, find the line corresponding to the Company Code
Fill in the current Fiscal Year and current Fiscal Period, and select one of the two options Allow Posting to Previous Period (Backposting) or Indicator: Disallow backposting after a change of period – you can’t select both as they are contradictory
Enter a customizing request
And the data should be saved and the issue solved, now allowing to proceed with material creation