A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Hi Cornejo,
To select External Content Type, the administrator should do some things first like: prepare the external data source, Configure Business Data Connectivity Services, Create an external content type etc. Thus, please contact your administrator first.
The following are some references for you:
External content types in SharePoint
Get started with Business Connectivity Services in SharePoint
In addition, since your question is related to development, we kindly suggest you post a new thread in our TechNet forum to get dedicated support. It is a special channel handle this kind of questions.
Appreciate your understanding.
Regards,
Zixuan