I haven't been able to obtain previous days EOD prices for a certain equity option? What is the best way to do it? I'd prefer to schedule the report in the GUI (to download via API once is generated). But another option would to get the data on demand. However I can not find a report that provides EOD fields or a way to create an on demand request to get this EOD prices
Doing this using an On Demand request is described in REST API Tutorial 7.
You can also do it using a scheduled request, that uses an Elektron Timeseries report template. REST API Tutorial 10 describes a scheduled request (but it uses a report template for tick data, not end of day data, but it is not a big deal to modify it).
For a rough view of the asset class coverage of individual extraction calls you can refer to the Tick History Data Dictionary. It is an Excel sheet. Open tab Field Descriptions, then select Elektron Timeseries in column A. This displays all the available field names in column B. Scrolling to the right, columns L-AF contain information as to which asset classes are covered.