saludos y bienvenidos. Y ahora, Ingles... Make sure that you are running the same version of calibre in windows and ubuntu. If you installed calibre from the ubuntu repository, you are not running the current version.* You do not need to import/export if your library is on the same drive that both OSs are or if you can mount the partition the windows calibre library is on. You can direct calibre (ubuntu) to use the calibre (windows) library. Make sure the windows partition ( the c drive) is mounted, then open calibre, click the calibre library icon in the toolbar. Select switch/create library, and when the dialog box opens, navigate to the calibre (windows) library folder as the new location. Check the box "Use a previously existing library..." click ok, restart calibre and your library should be there.
Should you want your calibre library on your ubuntu partition, simply copy the folder from windows, and all its contents, including the metadata.db and other non-book files. Tell Calibre that is where the library is, using the method above.
If you are using Ubuntu WSL rather than a separate partition, things are pretty much the same, although the library will be at /mnt/c/.../Calibre Library instead of /media/username/.../Calibre Library. There may be file ownership issues. e.g. a warning that /run/user1000/ has 0750 instead of 0700 permissions. The fix is "chmod 0700 /run/user/100" in the command line. This is temporary. to make it permanent add it to the .bashrc file, which is a hidden file in /home/username.
I've found the above method faster than export/import for moving libraries around.
*in the terminal ( command line) remove it, sudo apt purge calibre, then install the current version from here
https://calibre-ebook.com/download_linux ( sudo -v && wget -nv -O-
https://download.calibre-ebook.com/linux-installer.sh | sudo sh /dev/stdin) but before you run it make sure to install a missing library file. sudo apt install libxcb-cursor0 as directed on the download page.)