The author of MMM-MktIndex has updated a file that will get the module working again.
Go to this thread: https://github.com/thess/MMM-MktIndex/issues/2

I don’t think the trouble is on your end. My module is experiencing the same behavior. It was working fine and then recently no values are being displayed. I have read in some recent posts that Yahoo has done something with their Stock API so now any modules such as this one cannot retrieve any data. Hopefully the author of this module can create an update to get it functional again, I really like this module.

• Mike

I’m having the same issue with MMM-MktIndex module. I think it has something to do with Yahoo having switched off their Stock API or changed their API format or something like that a few days ago. All that coding stuff is over my head. I don’t know if the authors of MMM-AVStock and MMM-MktIndex are going to work on a solution or if one can even be done. I surely hope so, I have been using MMM-MktIndex on my magicmirror for a long time and really like the module and customizable ability of determining which stocks to track. I did see that there is an update to MMM-JaST (Just another stock ticker), here is link: https://forum.magicmirror.builders/topic/12507/mmm-jast-just-another-stock-ticker-new-version-2-x/147?page=15

-Mike