Are you referring to the function "Measure Distance Between Two Points"? If so, there is no button and never has been. The function works differently, depending on your OS.
iOS — Touch two points on the chart simultaneously;
Android — Long press (touch and hold) a point on the chart to bring up the "What's Here" menu, select "Routing" and then select "Measure from Here" then touch another point on the chart.
Windows — Hold down CTRL whilst clicking on a point on the chart, release CTRL and then click on another point on the chart.
In each case, a ruler will be displayed overlaid on the chart, showing the distance, bearing (T & M) to and from, time, fuel consumption and groundspeed between the two points. To get rid of the ruler, tap/click anywhere on the chart.
For further information, see the SD Manual, page 14.
Mike
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Samsung Galaxy Tab A8 – Android 11.0 & SD 3.16.13.0
Huawei P30 – Android 11.0 & SD 3.16.13.0
PC – Windows 10 (Home Ed) Version 22H2, Build 19045.4355, SD 3.16.13.0