Newtonian physics doesn't allow immovable objects.

If you have an acceleration and a force, you rearrange F=ma as m=F/a to find the last variable, mass.

If you try to say the object doesn't accelerate (a=0), you get m=F/0.

The illegal division by zero is the equation's way of telling you that you're talking nonsense.

(Note that if F=0, you get the indeterminate 0/0, which is a less committal kind of no-answer; the equation just can't tell you the mass because you've given it nothing to go on.)