SAP Training systems are different na dhave set of data loaded specifically to aid the progress of a course. Sandbox on the other hand is normally a copy of Development and used to check major upgrades of perform development work which may have impact on system which the management is not willing to risk on Dev. Many people confuse both as same and many companies use Sandbox for training and this causes normally no fuss, but it technically their function is different.

Check the rules for Posting Key for concerned field and GL account. Make sure that where it is set as "Suppresed" is set to "Optional".

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