“The share operation [share destination] has failed” might appear when sharing from a Final Cut Pro library located on an external storage device or when sharing to an external storage device. This can happen if the storage device isn't formatted properly. This can also occur when sharing to destinations other than the Export File destination. 

Check the format of the storage device. In the Finder, click on the storage device to select it, then choose File > Get Info. View the drive format in the General section of the window.

Local volumes, such as external storage devices, must be formatted as APFS or Mac OS Extended. Final Cut Pro libraries can also be stored on Xsan volumes and shared storage that use the SMB or NFS protocol. Some storage devices might be pre-formatted in unsupported formats such as FAT32 or ExFat, which you'll need to reformat. You also can't use drives used for Time Machine backups.

Before attempting to share again, do any of the following:

Learn what to do if you can't share to DVD or Blu-Ray from Final Cut Pro, Motion, or Compressor.