Connect a DC/DC converter to a virtual precision ground in KiCad
The third image shows that pin 1 and 2 of U3 (TLE2426) are effectively connected together. Actually the ERC should tell you that there is a conflict between power pins if the symbols are made correctly. To fix that issue, replace all +12V symbols with +6V and all GND symbols but the one connected to pin 1 of U3 (TLE2426) with -6V. In the end only pin 1 of U3 should be connected to GND for the here shown schematics.