Message type: E = Error
Message class:  FR - Messages for FI Reporting 
Message number: 123
Message text: Only customers OR vendors are permitted for certain special G/L trans.
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 FR123 ("Only customers OR vendors are permitted for certain special G/L transactions") typically occurs when you are trying to post a special G/L transaction that is not allowed for the account type you are using. This error is specifically related to the configuration of special G/L transactions in the SAP system, which are often used for transactions like down payments, guarantees, or other special financial arrangements.
Cause:
Account Type Mismatch: The special G/L transaction you are trying to post is configured to only allow postings to customer or vendor accounts. If you are attempting to post to a general ledger account or an account that is not classified as a customer or vendor, this error will occur.
Configuration Settings: The special G/L transaction may be configured in such a way that it restricts postings to only certain account types (customers or vendors).
Incorrect Account Assignment: The account you are trying Get instant SAP help. Sign up for our Free Essentials Plan.
Get instant SAP help. Sign up for our Free Essentials Plan. 
FR122 Only account types 'K' (vendor) and 'D' (customer) are permitted
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
FR121 Selection for dunning run & & was not carried out
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
FR124 Select an evaluation
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
FR125 No open items were selected during run &, &
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.