The optional function (Optional Feature) has to be installed for the SAP RFC support:

1. Start EEC
2. Open Help > Install new software....
3. In the Use field select the --All available sites-- or de.eplan.eec.install.optfeatures entry from the drop-down list.
4. Expand the EEC Optional Features path.
5. Select the SAP RFC Feature entry.
6. Click [Next >].

The function to be installed is only listed so that the selection can be checked.

7. Click [Next >].
8. Accept the license terms.
9. Click [Finish].

More:

Installation of the SAP Java connector