They have to be names Latitude spelled out. Make sure your x/y are in the same projection also! That is why I would do a xy event layer in arcmap then use that file to append.
CSV, TXT, and GPX files—ArcGIS Online Help | ArcGIS
Location fields in CSV and TXT files
To add a CSV or TXT file from a local drive or a CSV file on the web to your map as a feature layer or to publish a local CSV file as a hosted feature layer, the file must include location fields. Location fields can contain coordinate, address, or place information. The first row in the file must contain the location field names. Addresses can be stored in one or more fields. Coordinates must be in two separate fields.
If your file has coordinate fields, Map Viewer uses these fields to locate the features on the map. ArcGIS Online supports the following coordinate reference systems:
- longitude, latitude*
- latitude, longitude*
- Military Grid Reference System (MGRS)
- United States National Grid (USNG)