Learn Dutch: Wij zetten het kooktoestel hier, want er is daar een vlakke ondergrond. - We put the camping stove here, because there is a flat surface there.

The first er is the existential word used to say that something exists, as in er is (there is). The word daar provides the physical location (over there). English uses the same word twice when saying there is a flat surface there, but Dutch splits these roles. The word er introduces the flat surface, while daar points to where it is.

Could I use an unstressed er for the location instead of daar, making it want er is er een vlakke ondergrond?

No, Dutch strictly avoids putting two instances of er together. If you wanted to use an unstressed location, you would have to drop the second er entirely due to a rule called er-deletie, leaving just want er is een vlakke ondergrond. However, because the speaker is contrasting this spot with hier (here), they need to emphasize the location. To do this without creating a double er sequence, they substitute the locative er with the stronger daar.

Why does the verb is come right after er instead of moving to the end of the sentence?

The word want (because) is a coordinating conjunction, which means it joins two independent main clauses. Main clauses keep the standard word order where the conjugated verb is in the second position. If you had used the subordinating conjunction omdat, the verb would move to the end of the clause: omdat er daar een vlakke ondergrond is.