Hi @xyvic,
A 403 Forbidden error typically means a permissions issue. Do you have at minimum create and assign privileges on the Table you are trying to add a row to?
Are there any lookup columns that you are trying to set? If so, you'll also need Append To privileges on the table in addition to those permissions mentioned above, and Append privileges on the related Table of the Lookup you are trying to set.
As mentioned, a screenshot of your flow step would aid in troubleshooting.
---
Please click Accept as Solution if my post answered your question. This will help others find solutions to similar questions. If you like my post and/or find it helpful, please consider giving it a Thumbs Up.