Could not mount “disk2s2”. (com.apple.DiskManagement.disenter error -119930868.)

Any chance the drive is formatted NTFS? That is a read-only filesystem on the Mac. You might try reformatting by setting the partition scheme to MBR and format MSDOS. If that works, then try doing the Mac format. If it doesn't work, then reformat it on a Windows box using MBR and FAT32. Then take it to the Mac and reformat it for macOS.

Try reformatting the external drive.

Drive Preparation - El Capitan or Later

1. Open Disk Utility in the Utilities' folder.
2. After Disk Utility loads select the drive (out-dented entry w/type and size info) from the side list. 
3. Click on the Erase button in the Disk Utility toolbar. A panel should drop down.
4. In the drop down panel set the partition scheme to GUID. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
5. Click on the Apply button and click on the Done button when it is activated.
6. Quit Disk Utility.

Install El Capitan or Later from Scratch

If possible, back up your files.

1. Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
2. Select Disk Utility from the Utility Menu and click on the Continue button.
3. When Disk Utility loads select the target drive (out-dented entry w/type and size info) from the Device list.
4. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
5. Set the partition scheme to MBR.
6. Set the Format type to MSDOS.
7. Click on the Apply button, then wait for the Done button to activate and click on it.
8. Quit Disk Utility and return to the Utility Menu.
9. Select Install OS X and click on the Continue button.