The Studio Toolkit for Boxes Fold window gives an error that the Design cannot be folded because it doesn't contain panels or one of the panels cannot be selected as Base panel.
When using the Fold window to assign Cuts and Creases and try to convert these paths to an ARD file using the Fold button,
An error appears stating that there were no panels selected to fold and the window closes. Or,
An error pops up in the Fold Selection Dialog stating that the clicked panel cannot be selected as Base panel because of gaps or incorrect cut/crease assignments.
Make sure to click the Select All Cuts and Creases button in the Fold window to have your entire structure selected.
Use the Check button in the Fold window to check for double lines and gaps and correct if needed. Make sure that the complete outline of the structure has cut lines assigned and that all fold lines are correctly assigned as being creases and they touch other cut and/or crease lines.
If those do not work and the error still appears, check your paths (especially at cut path corners) to ensure the paths and not the strokes actually do match up point to point. Illustrator's Preview and Overprint Preview is unreliable since it displays the stroke weight, so it is advised to check the corners in Outline mode. Also note that our Check for gaps has a specific tolerance to be set in the flyout menu of the Fold window, so there may be corners that will still cause problems if not addressed.