Combine two shelly sensors into one, for energy dashboard

I am trying to combine several shelly sensors into one.
For example, I would like to display the room living room in the energy dashboard and not have all the sensors individually in the dashboard.

I have the helper function “combine the state of multiple sensors” the shelly devices together and selected the function sum.

but now when i select the sensor in the energy dashboard, i get the following error message:

Unexpected device class.
The following entities do not have the expected device class:
Last reset missing
The following entities have the state class "measurement", but "last_reset" is missing:

can someone please explain me what i am doing wrong?

what are the shelly sensors?
what do they report?
and how do they report it?