Can't import blender file into S4studio

Now this is perhaps a stupid question, but I'm very new to making mesh edits and everything was going fine until I had to import my mesh edit into s4studio. I was on the original mesh, clicked "import mesh" and then I got an error message that said I couldn't import my file because it was open in Blender. Blender was closed, I tried closing the s4studio program and opening it again, even tried restarting my laptop but still always the same message!
Can someone tell me if I'm doing anything wrong? Thank you