If you are in version 2.8 or higher, in Texture Paint mode with the Draw tool enabled, it is necessary to activate the Cursor option in Sidebar > Tool > Brush Settings > Cursor so that the Stencil appears in the viewport when the mouse is over the model.
But don't forget that in order to use the Stencil, you must first load a texture in Sidebar > Tool > Brush Settings > Texture and change the Mapping to Stencil. Click the Reset Transformation button if necessary.
If the Stencil does not appear in Image Paint, make sure that the 3D Viewport is in Texture Paint mode.