match_parent
is not allowed. But you can actually set width and height to 0dp and set either top and bottom or left and right constraints to "parent".
So for example if you want to have the match_parent
constraint on the width of the element, you can do it like this:
<TextView
android:layout_width="0dp"
android:layout_height="wrap_content"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintEnd_toEndOf="parent"/>