Using ER to export data to an excel is a much more complex way to get to a solution and not the intent behind ER. In ER there are a string off variables that causes errors/warnings. Best practice is to use Datamanagement, if you are dealing with Custom Tables, create a data entity for that. Secondly after having a data entity, you can create a OpenInExcel Template, and upload it into F&O, making it easy to access from Excel by anyone with access.
Your screenshot errors are unfortunately not specific to one step that is missing, but if I need to guess, it could be that your Tables/Fields classification is not set in your Excel Attachement.
I have simplified the use case for ease in understanding, but it is in fact on custom tables. I request experts to help in resolving the issue posted rather than providing alternatives.
if its just to export data why not use "Data management" workspace and use "Vendors V2" entity ?
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