Weight paint mode wont paint on the mesh

Question

So i downloaded this fully rigged model from blendswap,

and i made adjustments to the model in edit mode, i using the knife too made some cut to imitate that of a shoe to the model and thought that i would weightpaint on manually if the weights are not applied to the new vertices.

back in pose mode.. i pose the rig and confirm that the new vetrices did not get the corresponding weight of their neighboring verteces automatically, so i enter the weight paint mode to fix it and

the weight paint wont draw anything, none of the brushes work, i cant get to fix it!! here is how it looks

here is the .blend

Answer 1

To do this you need to select which vertices you need to paint first then paint them. So select the vertices as shown below using shift + right mouse click, then paint them using left mouse click. Updated .blend file can be found here.

Answer 2

choose the whole mesh and press Alt + M > Merge by Distance and remove all the doubles and try again.