A valid stock symbol or market index is required

WordPress has no such feature.

What plugin or theme is giving you this Stock Market widget?

Stock Market overview plugin. I can see stock listings from other countries but not mine.

My bad: I didn’t realize that was the name of the plugin.

And as you can guess by now, I’m not familiar with the plugin at all.

Please ask your question in the Stock Market Overview plugin’s own support forum so its author can help you:

https://www.ads-software.com/support/plugin/stock-market-overview/

Good luck!