Update app.py

app.py

@@ -345,7 +345,7 @@ def generate_certificate(certificate_model, first_name, last_name):
 
     im_temp = im.load()
     background = Image.new("RGB", im.size, (255, 255, 255))
-    background.paste(im_temp, mask=png.split()[3]) # 3 is the alpha channel
+    background.paste(im_temp, mask=im_temp.split()[3]) # 3 is the alpha channel
     
     pdf = background.save("certificate"+".pdf")