First, let's make sure we don't confuse the rest of our viewers. There is no such thing as a "watchlist scanner". There are custom watchlist columns and there are custom scans. We don't mix those terms here. I don't know who on earth started calling these "watchlist scanners" but there is no such thing on the Thinkorswim platform.
To be clear, a custom watchlist column displays values and colors based on various metrics. A custom scan returns a list of stocks that meet it's filter criteria.
Having corrected this, we can now dig a bit deeper into your issue. You see those cells in your watchlist that are displaying error codes instead of values? Well simply pick any of those columns and expand it so you can read the full error message. You will find there is a limit to how many values a custom watchlist column can display. Trying to use this for all the stocks in the NYSE, however useful you may have thought this may be, it is simply WAY beyond the capacity of this tool. Never in my wildest imaginations would have expected this to work.