I’ve never had an issue saving anything to LibreOffice.
You don’t save files to LibreOffice. You save files to storage devices, which may be local to your system (disk, USB devices) or remote (NAS server, Cloud services, or other computer system).
it will not allow
What is it? As stated above, it isn’t LibreOffice but your system in combination with your storage device (whatever it is; you didn’t specify).
A window pops up showing:
When does it pop up and which application causes this to pop up? Again - this couldn’t be LibreOffice since it is not a program to download files from the internet but load files from storage devices.
C:/Users/user name/Documents/document name.odt File not found. Check the file name and try again.
Did you follow the advice given and have you checked C:/Users/<user name>/Documents/
using File Explorer (aka Windows Explorer), whether the file exists?