When we work with the Planning calendar make sure the material master is having the right master data set up.
IN some instances we may use a Lot size with planning calendar , but forget to maintain the planning calendar in the material master.
In sufficient period in the assigned planning calendar.
So during such instance we fill face an error like below.
Message no. 61123
Error determining plannin
In such case do check the material master and then the planning calendar.
Let me know if it helped for you .