but I have 15gb free

Bazzu85
No, you don't. You're running your hard drive at redline already - your PC is trying hard to tell you that your drive is damn near full and your system performance is in danger of being affected - and StreamFab is seconding that. You only have 14.7GB "free" to fill up if you're a big fan of blue screen crashes.
You need to delete some files you've saved on your drive, run disk cleanup to delete all temporary files, empty your trash, maybe delete software or bloatware you never use, and generally make some space on your drive to keep your system running optimally - and not just to run StreamFab, which needs to use more drive space than the downfile file size to create the download.
Once you delete some files and free up enough drive space to get well above redline, see if you still get that error message.
You can read more about the issue here:
https://forum.dvdfab.cn/forum/softwa...temp-directory