Message type: E = Error
Message class:  TXRG_POSTING - Tax Register Posting Message Class 
Message number: 604
Message text: Tax Code is empty
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 "TXRG_POSTING604: Tax Code is empty" typically occurs during the posting of financial documents when the system expects a tax code to be specified, but none has been provided. This can happen in various transactions, such as invoice postings, where tax calculations are necessary.
Cause: Missing Tax Code: The most common cause is that the tax code field is left blank in the document being posted.
Configuration Issues: There may be issues with the configuration of tax codes in the system, such as missing tax codes for specific transaction types or incorrect settings in the tax determination procedure. Document Type Settings: The document type being used may not be configured to allow for tax codes. Master Data Issues: The master data for the vendor or customer may not have the necessary tax information set up.
Solution: Check Tax Code Field: Ensure that the tax code field is filled in the document 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. 
TXRG_POSTING603 Transaction key is invalid
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
TXRG_POSTING602 VAT check has failed
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
TXRG_POSTING605 Tax Type is empty
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
TXRG_POSTING606 Posting Indicator is empty
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.