If myTracks does not record any meaningful or very imprecise tracks on the iPhone, there can be various reasons. The first thing you should check is the Recording Settings. To do this, tap on the (REC) at the bottom left and then on Settings. The Tracking Mode should be set to GPS and the Accuracy Threshold to Adaptive. These settings should ensure that myTracks records track points even if the GPS accuracy is low.
The setting Record in motion only ensures that track points are only recorded when the iPhone is actually moving. If the iPhone is moving very slowly, this may cause problems. If you have problems with the recording, then deactivate this setting as a test.
Finally, you should check whether myTracks has general access to the iPhone's location. To do this, open the iOS Settings app and tap on myTracks. Check if Location is set to Always or While Using the App.
It can also happen that a recording is stopped without further action. Why this can happen and what you can do about it is described in this article.