I want to access feature service from ArcGIS Enterprise portal by Esri ArcGIS Portal Feature Service Reader/Writer (FME)
Greetings @RomuldKiobya
What you want is possible and has been documented by both Esri and SAFE.
From SAFE, please see Esri ArcGIS Portal Feature Service Reader/Writer and also the tutorial Tutorial: Getting Started with ArcGIS Online and Portal
Form Esri, please check Creating an FME Web Connection for your Enterprise Portal and, if you happen to be using SAML logins, Connect to ArcGIS Enterprise and ArcGIS Online using SAML logins in FME/Data Interoperability
Also, you may want to check the following excellent videos:
A Guide to Integrating with ArcGIS Enterprise and GIS and Data Integration with ArcGIS and FME
I hope the above will be of help. Have fun,