Some time after Rebels but before Ahsoka (the show, that is), Ahsoka took Sabine as her padawan despite Sabine not being Force-sensitive. Sabine eventually quit due to frustration over not getting anywhere, but had actually picked up a rudamentary ability to sense things through the Force even if she couldn't actively use it. After all, the Force is in everyone, as the Jedi like to mention. During the Ahsoka show, she learns how to use it actively in a die-or-fly situation, but she's still at a very basic level.

Basically, Ahsoka and Sabine proved normals could learn to use the Force with a combination of a thorough understanding of the underlying principles and stubborn determination. The threat of death may have also played a role.