First off, thank you for taking the time to provide the screenshots and detailed bug report. :)
When you select the block (tab+left click it) and hover over the red Water (Purified) job in the tooltip, does it show you what the error is. It should say something like missing skill or missing item.
My best guess is that they can't reach the coal that's required for the job. Could be somewhere in a cave/mine and the settler just can't path to it.
If the coal is available nearby, try remove the job from the cauldron (opening the job menu and right-click the cauldron.)
If none of that helps, I'd love to take a look at the save file. You can email it to me at [[email protected]](mailto:...
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