It may be necessary to click that slider if the NVMe has old data on it and make sure to select ALL partitions listed before clicking Start Flashing.
Using Linux titantools Version 2.0.1-Rc for linux (Not available on Github, but I got a prompt when running 1.0.35-beta) it was not possible for me to select all 3 options. Generally option 2 (partition_flash.json) was not selectable. Using no option for the Configure Partition File Slider, or only one the Jupiter_V1.0 would not boot, even after flashing successfully.
I got it working after selecting partition_2M.json and partition_universal.json.
This selection resulted in the following partitioning:
The Ubuntu Image I used: milkv-jupiter-ubuntu-23.10-desktop-k1-v1.0.9-release-2024-0719.zip