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.