@tonyzhang0510
Well, I'm not sure at what step you have had an issue, so I'll provide you my steps.
In SharePoint - created a Choice column on a list. Added your first 6 choices as seen in picture.
In PowerApps - created a new App. Added a datasource to the SharePoint list.
- added a dropdown control to the app.
- set the Items property to Choices(mySource.Column) - same as your Choices(Form.Company).
- added a ComboBox with the same as the above.
Both worked fine.
I am using Chrome for the designer. Just thought I would mention that in case you are dealing with a browser issue possibly.