Sometimes the DEM has pixels with nodata. These voids need to be interpolated.
1. In the Algorithms panel double click on the Fill nodata tool.
2. Change the Description to Interpolate voids in DEM. Choose as Input layer the output from the Reproject and clip DEM algorithm. Increase the Maximum distance to 100 pixels.
3. Click OK.
Your model should now look like the figure below.
We can proceed with the next step.