Your question seems quite basic, so I advice you to have a look at some tutorials (like Klas Karlsson's QGIS 3 for Absolute Beginners), books (like Kurt Menke's Discover QGIS 3.x) and the QGIS documentation - I added a few links below. There are many more great resources out there.
Excel files, CSV data, KML etc. are file formats that have limited editing functionality in QGIS: you can't edit them or just in a reduced form.
How to solve it
To edit geometries (like polygons) you must use a GIS format like Geopackage or Shapefiles. Create a new, empty layer, toggle editing mode and draw your polygons. You can pre-define a number of attributes that you can fill in when you finish drawing a feature or you can add attributes later using the attribute table and field calculator - or even filling in the table manually.
If you already have attribute data in an excel file, you can load that to QGIS and make a table join, joining the attributes to the polygons. However, you need a common, unique identifier that tells QGIS which line in the excel sheet should be connected to which polygon.
The attributes in this way are only joined to the vector layer and you can't edit them. To cahnge values, you still must do it in the excel file. To be able to edit the attributes, export the joined layer to a new (Geopackage) file - this way, you'll get "real" attributes you can edit.