Function cannot be executed at present (please wait)

What causes this issue?

The screens for sales and profit planning are generated by the system
when the transaction is run.
Online, the system can generate 1000 screens at any one time (for all
the users of the transaction). If more than 1000 are required, the next
user must wait until another user has ended the transaction.
As the numbers of screens involved in batch-input mode must be known in
advance, it is not possible to run the planning transaction
simultaneously with several variants in batch-input mode.

Error message extract from SAP system. Copyright SAP SE.