Message type: E = Error
Message class:  EDOCUMENT_IN_EINV - eDocument Inda eInvoice 
Message number: 026
Message text: Field 'Gstin' is not filled under node 'DispDtls'
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 EDOCUMENT_IN_EINV026 indicates that the field 'Gstin' (Goods and Services Tax Identification Number) is not filled under the node 'DispDtls' (Dispatch Details) in the electronic invoice (e-invoice) data structure. This error typically arises when generating e-invoices in compliance with GST regulations in India.
Cause: Missing GSTIN: The most common cause of this error is that the GSTIN field is not populated in the relevant data structure when creating the e-invoice. Incorrect Configuration: There may be issues with the configuration settings in SAP related to e-invoicing or GST. Data Entry Errors: The user may have missed entering the GSTIN during the invoice creation process. Integration Issues: If the data is being pulled from another system or module, there may be integration issues leading to missing data.
Solution: Check Data Entry: Ensure that the GSTIN is correctly entered in the relevant fields before generating the e-invoice. This can usually be found in the customer master data
 Get instant SAP help. Sign up for our Free Essentials Plan.
Get instant SAP help. Sign up for our Free Essentials Plan. 
EDOCUMENT_IN_EINV025 'Pin' & is not valid under node 'BuyerDtls'
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
EDOCUMENT_IN_EINV024 Field 'Stcd'(State Code) is not filled under node 'BuyerDtls'
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
EDOCUMENT_IN_EINV027 'Gstin' & is not valid under node 'DispDtls'
Self-Explanatory Message Since SAP believes that this specific error message is 'self-explanatory,' no more information has been given.The majority of...
EDOCUMENT_IN_EINV028 Field 'Nm'(Name of the Company) is not filled under node 'DispDtls'
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.