You didn't read the Placement Generator's description closely enough.
A Sphereworld needs 20 thousand kT of cable -- TEN cable components. A ringworld needs 10 thousand kT of cable -- FIVE cable components.
The placement generator you built was working fine, it found the cable and the plate you had built. To build your ringworld, it was comlaining about the OTHER
FOUR of
EACH that were missing.
