Can I substitute bread flour for all purpose?

You can, but the bread may turn out, heavy, or chewy.

For the bread maker I have, it says not to substitute bread flour for all-purpose.

Bread flour has a higher gluten content than all-purpose. This is usually recommended for sandwich breads and such, but zucchini bread is more like a pastry-bread. The higher gluten is likely to make the bread tougher and heavier than the bread would be with all-purpose flour, as @SpatzieLover suggested. You can try it, it probably won’t be terrible. Bread flour in cookies or cake would NOT be good, though.

@hannahsugs Bread flour in cookies or cake would NOT be good, though.

;D Unless you like to make your own hockey pucks or shot-puts ;D

My guess is that in a quickbread where minimal stirring is often advised, it will make little difference.

Take a risk, experiment! What’s the very worst thing that could happen…..now that wasn’t so bad was it?