So you are:
- Uploading a file to ArcGIS Online
- Going to an existing Hosted Feature Layer Item Details
- Clicking 'add and update features' to insert new feature and update existing.
Can you confirm the workflow? What is the input file type?
Generally this will occur when there is a misalignment between the uploaded file and the target layer. For example, you have a string field in a CSV that could exceed the length of the existing layer, or maybe it's a string value trying to be inserted into an existing integer field.
ArcGIS Online will check a sample of uploaded records to compare but it won't compare all until it executes the request. The onus is on you to ensure the uploaded data correctly matches the target schema. You can clean up the data or try to field map it - which may be easier in ArcGIS Pro.
More information on the considerations:
https://doc.arcgis.com/en/arcgis-online/manage-data/manage-hosted-feature-layers.htm#APPEND
Some examples of misaligned data types with CSVs and how it can cause issues:
https://support.esri.com/en-us/knowledge-base/problem-generic-errors-from-uploading-and-published-cs...
You may be able to get a more specific error by monitoring the web traffic and seeing the server response to the request.
