Welcome to the forum.
Getting boxes and other tabs and slots to fit can be a challenge, and it is so disappointing to print everything and end up with nothing. Ask me how I know.
It is important that you note what size material the designer says the files are for and that your material matches that thickness. If things are just a little tight, you can sand the tabs a bit or sand the openings to make them larger. Sometimes a rubber mallet is necessary. Since your purchased the files, you can’t share them here so someone can look at them, unfortunately.
Have you successfully made any of the boxes? Sometimes I print just one slotted piece and one tabbed piece to see if the fit is too loose, too tight, or just right. Most of the things I have made are for medium proofgrade sized material - 1/8". Have you tried any of the boxes in the Free Laser Design section of the forum? Many are great designs and are made by Glowforge users for other Glowforge users. One of my favorites is the simple hexagon box. Plain Hexagon Box