Symptoms:

LucidLink fails to mount your file space and in the meanwhile you see a dialog window saying that the LucidFS (Team ID "3T5GSNBU6W" Bundle ID  "com.lucidlink.lucidfs.filesystems.lucidfs") kernel extension developed by Benjamin Fleischer needs to be allowed explicitly.

Solution:

Click on “Open System Preferences” button in the dialog, or if you closed it go to  "System Preferences > Security & Privacy > General".
Then click the Allow button to enable the FUSE extension

Possible Problems:

• No "System Extensions Blocked" dialog and no Allow button in System Preferences. If this is your work machine, there is a chance that the software on it is managed by the IT department in your organization. Please check if the machine is configured for MDM (Mobile Device Management) and if so - contact your company's IT department for further assistance. To see if MDM is enabled, open the Terminal app, then type

profiles status -type enrollment

Enrolled via DEP: No
MDM enrollment: No

• No Allow button in System Preferences. If you opened the System Preferences more than 30 minutes after you saw the “System Extension Blocked” dialog the Allow button will have disappeared from system preferences. Try to mount a Lucid file space again and then go to System Preferences straightaway.
  
• Clicking the Allow button does not work. For security reasons only trusted Apple hardware devices can be used to click on the allow button. If you are using an external mouse from another vendor it might not work. Additionally, apps that offer extended user experience might be capturing the user input and transferring it to the system. This turns hardware clicks into software clicks and macOS does not accept those on the Allow button.
• Allow button is locked down. When you click the Allow button this approves the Apple Developer Team ID for the FUSE (osxfuse) kernel extension (kext) author. In enterprise macOS deployments this may be locked down. Administrators may deploy kext whitelists in bulk, only allowing kernel extensions associated with specific developers. The osxfuse kext is signed by Benjamin Fleischer - Team ID: 3T5GSNBU6W.

Reason:

Latest updates in macOS High Sierra require explicit approval for loading of third party kernel extension modules.
Lucid leverages LucidFS (Team ID "3T5GSNBU6W" Bundle ID  "com.lucidlink.lucidfs.filesystems.lucidfs") kernel extension, developed by Benjamin Fleischer. Therefore, in order to mount a Lucid file space users need to approve this extension.

External resources:

Apple Technical Note: https://developer.apple.com/library/archive/technotes/tn2459/_index.html#//apple_ref/doc/uid/DTS40017658
Apple Developer Forums Thread: https://forums.developer.apple.com/thread/79172
Related OSX Fuse Issue: https://github.com/osxfuse/osxfuse/issues/437

If you receive Failed to mount the filespace: Cannot prepare mount point with elevated permissions: child exited with 1. Please view article Failed to mount the filespace: Cannot prepare mount point with elevated permissions: child exited with 1