Any way to make a checkbox system to write out a list?

I am trying to set up a system on Google Sheets, where checking a checkbox will write out a list, for example:

Clicking on a checkbox AB will add "AB" to 1 single cell. Clicking on CD, will add "CD" on the same cell, after AB, unless AB was not checked, in that case CD will be first. https://gyazo.com/33dc413f0789f5408ffb1f4b6aee4445

Ideally, it would look like this https://gyazo.com/b3b0fa35d0cc12cff82a75f129da3326 but I don't know what's the most efficient way to do this.

I tried using a =IF formulas, which worked for marking a single project, but doing more than 1 results in an #ERROR