I am working on creating a storage account and a nfs file share which need to be mounted on on prem linux server. which I have done successfully. I can read and write from onprem server.
the issue is I am not able to upload any file from the azure portal and the file share is using the private endpoint
here is the error --> ("message". "load failed". "isError"true)

It could be a network issue.
If your on prem linux server is accessing the file share via the private link. But when you try to access the file share using a browser on your machine via the portal the traffic goes via the internet.
To test this try to access the portal from a browser on your Linux server.
It could also be a rights issue.
Does the user that is logged in to the portal have permission to write to the file share?