STL Extraction problem

Hello, sorry if I am not posting to the right section of the forum, as this is my first thread. I have made a bracelet with Blender and used a boolean operator (difference) to cut a letter inside of it. However, I am encountering errors when exporting and trying to slice it with Slic3r (in order to 3d-print it). The error says that (-) - subtraction - was not defined in some line L. What can I do to fix it?

I am an amateur blender user.
Thanks in advance! :smiley: