Suppose by mistake you closed the period and want to reopen it.

Use transaction code MMPI.

Enter the period you want to open sequentially.  For e.g. the current period is 10, and you want to be open for the 7th one, then you have to open period like 9th, 8th and 7th. After making the correction you have to closed the period again sequentially.

There are no major-minor impact on the system, but it is better to do this once only.  Regular practice for this transaction may be harmful.

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