Lasso tool

I have a current version of Photoshop (2018) & I am a long term user, while editing a photo I selected the Lasso tool, but when I highlighted the area I wanted to change, the following message appeared "Warning : No pixels are more than 50% selected. The selection edges will not be visible."

This then renders the tool as inoperative & it also stops the Brush Tool from working. By restarting the program, I can get the Brush Tool working again, but a soon as I try to use the Lasso Tool, I get the same message as shown above & both Lasso & Brush stop working again.

I also had this problem with an earlier version of Photoshop, but I fixed this buy installing the current version. I assumed my problem was a fault in my old version, but now it's back on the 2018 version & I have no idea how to fix it. HELP please!