Got it, thanks for explaining! So the issue is that Clay doesn't allow merging 'Find People with Filters' back into the same table that's being used as the source (the error is basically Clay's way of preventing a circular import).
The fix: instead of using one table for everything, set up a workbook with two separate tables, one for Companies and one for People. Run 'Find People with Filters' on your Companies table, but send the output to the new People table instead of merging it back into Companies. That should resolve the error.