Wrong key. Copy-paste it from your dashboard instead of typing it.
That key is for a different game. You need a Breachers key.
Restart the game and try again with the correct key.
In-Game Issues
Something else is using port 8777. Close whatever is on that port and retry.
You need to be in a match, not the lobby. Features only activate once teams are assigned.
Make sure you assigned a button in the web UI dropdown for that toggle.
Restart the game and try again. If it keeps happening, check that your Windows TEMP folder isn't full or read-only.
The game probably updated. Join Discord and open a ticket so we can check it out.
Before You Launch
Go to Windows Security > Device Security > Core Isolation and turn off Memory Integrity. Reboot after. If it's greyed out or won't turn off, you need to disable virtualization in your BIOS. Look for any of these: Intel VT-x, Intel VT-d, Intel Virtualization Technology, AMD-V, AMD SVM, SVM Mode, Vanderpool, Hyper-V, or just Virtualization. Disable it, save, and reboot.
Same page as Core Isolation. Turn off the Microsoft Vulnerable Driver Blocklist toggle and reboot.
Add the entire Eclipse folder to your antivirus exclusions. In Windows Defender: Settings > Virus & Threat Protection > Exclusions > Add Folder.
Open Medal, launch Breachers, and make sure Medal is actively clipping the game before you run Eclipse.
Right-click the Eclipse loader and select "Run as administrator". It won't work without admin privileges.
Troubleshooting
Your antivirus is tampering with the file. Exclude the Eclipse folder from your AV and redownload.
Medal isn't clipping the game. Open Medal, make sure it's set to clip Breachers, and that the overlay is showing in-game before you run Eclipse.
Reboot and try again. Don't run two instances at the same time and don't close it mid-process.
VBS is still on or needs a reboot. Disable Core Isolation and the Vulnerable Driver Blocklist, then restart your PC.