Hi Kiran,
I just faced the same issue with BAPI_BUS2054_CHANGE_MULTI
I fixed it playing with conversion exits, use this CONVERSION_EXIT_ABPSN_INPUT to load the data to BAPI's WBS ELEMENT field.
About the issue with BAPI_PS_PRECOMMIT you just need to be sure that all mandatory fields are filled in for T-Code CJ20N.
These 2 errors are just data issues.
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Error # 1 : Final check of the project definition and the WBS elements: Error
Error # 2 : Fill in all required entry fields
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I hope this information helps.
Regards,
Ricardo