Start with the accounts involved.   Are the accounts actually intended to be cost/revenue elements?  If they should be CRE’s, should the code part ‘J’/Projects be ‘mandatory’?  If the Project code should be mandatory, verify the correct mapping is listed in the M49 & M184 posting controls/details.

Hello,
MPL is a voucher transferring transactions from inventory. If it was created successfully, it means hat posting issue has not been identified at that time - usually all posting errors are identified before voucher is created. Was it recent setup change? 
Anyway, you cannot mofify MPL voucher postings - they are created automatically. All you can do is to try change account settings:

• if project id is present in posting line together with activity sequence, you are blocked and need to call support for back-end data repair
• If project id is present, but activity sequence is not, perhaps setting account as excluded from project follow-up will be an option (I suspect code part J containing project cost element is already set to optional or even blocked for this account).

Hi,

The reason for this error is basic data set up in posting control is missing. You have to have postings set up for M49 and M184, code part name J(which is the project cost revenue element). But after the MPL voucher is created you cannot edit it.

There are couple of workarounds to overcome the situation:
1) temporarily change the restrictions on code part demands and handle it using a manual correction voucher
2) Data Repair (Since this is in cloud environment and R&D has classified this as a user error, you have to create a ticket for consulting organization to get this resolved)