Studio Toolkit 12.x does not support importing/ opening .bag files which were created in Studio Toolkit for Flexibles.
Esko switched from .bag files to Collada files because:
Collada is the most commonly used 3D format
Collada has more input/output advantages
In case you want to make a composition that is featuring bags files that were made previously (in the old Studio Toolkit for Flexibles app), then there is a workaround. Use Studio Designer in Adobe® Illustrator®, ArtPro or PackEdge to convert the .bag file to a Collada file without graphics but with printable part. You can convert the .bagfile to a Collada file by following the steps given below:
Place your .bag file as a new structural design file in Adobe® Illustrator®: File > Structural Design > Place File....
Use the Studio Designer plugin for Adobe® Illustrator® to export your file to a Collada file.
Open the fly-out menu in the Studio Designer window.
Export your file to the Collada Archive (.zae extension).
Make sure you enable the Keep Printable Parts option.
Now, you can open the Collada file in the Studio Toolkit: File > Open....
If you wish to make some modifications to .bagfiles that they made previously, then:
Either rebuild the bag in the Toolkit, using the same parameters.
Or open and tweak the bag with the old toolkit for flexibles (which would still work with your new licenses).