Why is the brush not working in Unity?

Question

Unity

Good afternoon-evening-night, dear experts!
I decided to try to work with Unity.
I downloaded video tutorials, began to slowly master.
I made a workspace, drew mountains in it, decided to paint it, downloaded and selected a testura (grass), clicked - everything was painted automatically, only some other square was formed in the work square, with an area of ​​​​% 30 from the main one, with some differently shaped spots , which are not painted over, a kind of swamp. There was nothing of the kind in the video.
Then I decided to paint the mountains so that they differed from the ground, chose a different texture, clicked - these spots were painted over by themselves, but the mountains - I put a brush on them, click, click, click - nothing happens ... At the same time, there was a shadow in the textures , I tried it - it lay down on my mountain without any problems, but I just can’t paint the mountain with another grass ..... I’ve already tried several textures, to no avail ... I tried to ask the author of the lesson, but he was busy doing something , "does not give consultations" .. Maybe there are good people here .... Thanks in advance !!!!