The error code is indicating that a field name is a reserved keyword. Some words like 'DATE', 'DELETE' or 'TRUNCATE' should not be used as field names as they are incompatible with databases. There is a strong chance that you have such a keyword in your layer.
ArcGIS Online would have a different underlying database to ArcGIS Field Maps. Some keyword restrictions would only manifest in one or the other, not both. General best practice is to avoid a list of keywords to ensure you don't have issues when data is moved around e.g. to a SQL server, file geodatabase, shapefile...
Take a look through your fields. Here are a couple of lists that may help:
https://support.esri.com/en/technical-article/000010906
https://pro.arcgis.com/en/pro-app/2.8/tool-reference/tool-errors-and-warnings/010001-020000/tool-err...
Please don't forget the kudos 🙂