Is this a recipe you’ve come up with on your own? A number of things could make it too crumbly, but the use of a cake mix with the addition of flour makes us think it’s got too much flour to hold together. Also, biscotti are always baked twice, so maybe you’re just expecting too much of once-baked biscotti.
Generally, the loaf is baked for a half hour or so, then when it’s cool enough to handle, the cookies are cut, and put in the oven for 15 minutes or so longer (re-cooking on them on a wire cooling rack saves you from having to turn them over) until they are brown and crunchy. If you only cooked them once, it would not be surprising that they didn’t perform as expected.