Ah, version is likely why that setting isn't available. The link I posted is for the web-based Operations Dashboard for ArcGIS Online and Enterprise 10.6+. The number prefixes setting was introduced last year:
The Indicator, Gauge, and Charting Dashboard elements support unit prefixing. Prefixing can be quite useful when indicating a multiple or fraction of a unit, such as “kilo” for thousand. Until now, Dashboard would simply apply the standard metric prefix symbol. However, with this update, when you access the Dashboard Settings, and click on the General tab, you can set the symbol used for the prefix. In the example below, you will see that the standard metric prefix for billion is G, for “giga”, but can be overridden to B for “billion”.
The blog post from when the new version of Operations Dashboard was introduced:
The support page for the earlier version of Operations Dashboard: