Are you using a Cricut Explore or Cricut Maker machine? You should use our solid score lines! And you need to use the "Attach" button.
Linetype
After uploading/inserting an SVG file into the Cricut Design Space that contains solid score lines, you'll need to select the score lines and change their Linetype to "Score."
Attach
Then, select both layers of the file and click the Attach button.
That's it!