Message type: E = Error
Message class:  CNV_20306_CHECK - Message Class for package CNV_20306_CHECK 
Message number: 021
Message text: All provided source controlling areas exist
Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of messages in the SAP system have a message text, however this is frequently insufficient to comprehend or resolve the problem.
To make things easier, more detailed information is frequently added to describe the issue, how to fix it, and the necessary steps or configuration modifications.
Unfortunately, there isn't any extra information in this error notice.
First, use our AnswerBot below to get a possible cause and solution (requires a premium subscription).
Also, review the in-depth Common Questions & Answers listed below; you could discover a solution there or be able to connect with others who have faced similar challenges.
You can also try searching the SAP support portal (support.sap.com) but you need a special user ID to access it. It is possible that an SAP support note exists that provides additional details about the mistake or even steps for fixing it.
The SAP error message CNV_20306_CHECK021 typically occurs during data migration or conversion processes, particularly when using the SAP S/4HANA Migration Cockpit or similar tools. This error indicates that there is an issue with the controlling areas provided in the migration data.
Cause: The error message CNV_20306_CHECK021 suggests that the system is unable to find one or more of the controlling areas that have been specified in the migration data. This can happen due to several reasons: Non-existent Controlling Areas: The controlling areas specified in the migration data do not exist in the target system. Incorrect Configuration: There may be a mismatch in the configuration of controlling areas between the source and target systems. Data Entry Errors: There could be typographical errors in the controlling area codes provided in the migration data. Authorization Issues: The user executing the migration may not have the necessary authorizations to access the controlling areas.
Solution: To resolve the error CNV_20306_CHECK021, you can follow
 Get instant SAP help. Sign up for our Free Essentials Plan.
Get instant SAP help. Sign up for our Free Essentials Plan. 
CNV_20306_CHECK020 Source controlling area &1 does not exist
What causes this issue?  A source mapping value does not exist in the system. To run the transformation solution, all source mapping values must exist...
CNV_20306_CHECK019 No controlling area split found
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
CNV_20306_CHECK022 Target controlling area &1 not in source mapping
What causes this issue?  In the mapping, you entered a target controlling area that exists. However, the mentionend controlling area was not supplied ...
CNV_20306_CHECK023 No target provided for source &1; mapping is not complete
What causes this issue?  The provided mapping is incomplete. A mapping line contains a source value (&V1&), but not a target value.System Resp...
Click on this link to search all SAP messages.