To make use of Temperature Conversion you need to make sure Unit Conversion is set up, both in general and for the recipe where you want to use it.
If a recipe has a second unit system set, the unit conversion buttons will show, allowing visitors to switch between US Customary and Metric units.
To have the temperature unit switch as well, make sure to enable "Temperature Conversion" on the WP Recipe Maker > Settings > Unit Conversion page:
If you set it to "Changes with Unit System", check if the correct temperature unit is associated with the correct unit system:
In general you'd want °F for US Customary and °C for Metric.
Take note that temperature conversion will only work for temperatures that you've added using the temperature shortcode.