If you're using a device that does not support (or you don't have) an active pen, make sure you have not selected something other than Finger as your Primary input method. Go to the Squid settings, tap Input methods, then tap Primary input method and choose Finger. When Finger is chosen, a single finger touch always performs the current tool. When Active pen is chosen, or the name of the active pen for your device (e.g. S Pen, DirectStylus, etc.), a single finger touch always performs the action selected in the Finger action setting rather than the current tool (by default, this is the stroke eraser tool).
Select Finger as the Primary input method to allow the single finger touch to perform the current tool, or change the Finger touch setting to Current tool.