Message type: E = Error
Message class:  SADL_GW_DT_CDS - Messages CDS Checks 
Message number: 056
Message text: Field &1 must be exposed because it is used in annotation &2 at field &3.
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 SADL_GW_DT_CDS056 indicates that a field in your Core Data Services (CDS) view must be exposed because it is referenced in an annotation. This typically occurs when you are using annotations to define certain behaviors or properties of your CDS view, such as UI annotations for Fiori applications, and the field in question is not included in the output of the CDS view.
Cause: The error arises when: A field is referenced in an annotation (e.g., @UI.lineItem, @UI.selectionField, etc.) but is not included in the list of fields that are exposed in the CDS view. The CDS view is expected to provide certain fields for UI rendering or other purposes, but those fields are not part of the SELECT statement.
Solution: To resolve this error, you need to ensure that the field mentioned in the error message is included in the CDS view. Here are the steps to do that: Identify the Field: Check
 Get instant SAP help. Sign up for our Free Essentials Plan.
Get instant SAP help. Sign up for our Free Essentials Plan. 
SADL_GW_DT_CDS055 &1: The type of element &2 must be compatible with data element &3.
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
SADL_GW_DT_CDS054 CDS views cannot be analytical and transactional at the same time.
What causes this issue?  The CDS view is marked as transactional and Analytics.query. Example: @Analytics.query: true @ObjectModel.transactionalProces...
SADL_GW_DT_CDS057 Where-condition in consumption view can only have operator: &1.
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
SADL_GW_DT_CDS058 Where-condition of view &1 does not match filter of association &2.
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
Click on this link to search all SAP messages.