I'm using Corel Painter 2018 and have encountered an anomaly in the paper. In the center of the work area there appears to be a scribbled out region that won't accept any paper texture or colors. This anomaly appears across all the layers, regardless of the brush type or paper texture selected. There doesn't appear to be any way to remove it. I thought it might be the result of something I did when setting of the drawing. Is there some sort of "scratching tool" I may have activated?
Try a new blank canvas, fill with a color, go to Effects, Surface Control, Apply Surface texture, Using Paper and see if there are any anomalies.
Your screenshot appears to be from a brushstroke- attempt to find the layer it's on and try the eraser, also make sure Impasto is turned off.