I have found this question and the solution was this code :
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="rectangle" >
<solid android:color="#ffffff" />
<corners
android:bottomLeftRadius="8dp"
android:bottomRightRadius="8dp"
android:topLeftRadius="8dp"
android:topRightRadius="8dp" />
</shape>
This code doesn't work on my PC (no rounded corners). And you ? There has been changes ?
EDIT 1: I have an error when I build the project : AAPT: error: XML or text declaration not at start of entity. (finally corrected : stupid mistake from me)
EDIT 2 : The error is now : The following classes could not be found: - corners (Fix Build Path, Edit XML) - shape (Fix Build Path, Edit XML) - solid (Fix Build Path, Edit XML) Tip: Try to build the project.
XML layout :
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent">
<shape
android:shape="rectangle"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent">
<solid android:color="#ffffff" />
<corners
android:bottomLeftRadius="120dp"
android:bottomRightRadius="120dp"
android:topLeftRadius="120dp"
android:topRightRadius="120dp" />
</shape>
</androidx.constraintlayout.widget.ConstraintLayout>
SOLUTION :