Allow yourself to produce junk
Some days are just like that.
You know those days... you pick up a pencil or a brush or some other tool to work with. You sit down (or stand up) and start your work. And it looks like crap. Keep going.
You erase, color/paint over it. And it looks worse. Keep going.
You're tired, you're hungry. The freezer needs defrosting. A million other things you have to do fill your mind. Definitely not in "the flow". Keep going.
Make it to that 15-minute (or whatever) goal for the day.
Then close the sketchbook, put the tools down, and move on.
Every doodle will not be a Picasso. Every stroke will not be a Rembrandt. Keep going.
Allow yourself to produce whatever you produce. The more you do it, the better you will get. Move on and keep going!