How can I fix this error ?
That’s not easy to say.
It could be that there is no disk space available or you don’t have write permission.
You could check the messages.log file, this might give more information.
You can find it by selecting from the menu Help / Show Log Directory.
Make sure to reproduce the error first and then check the messages.log file.
The same was asked here yesterday: Sentinel-1A/B
Maybe the reason for the error lies in the spaces in your output path.