When you are ready to try again......and there are no guarantees here, since El Cap has some weird things going on with Time Machine......
Click anywhere on the Mac open desktop so that the Finder menus are displayed at the top of the screen
Click the Go menu
Click Go to Folder
Enter the following in the search box
/Library
Open the Preferences folder
Locate the following file.......com.Apple.TimeMachine.plist
Click to highlight the file
Click the Delete key on the Mac
Restart the Mac
Open System Preferences (gear icon on the dock)
Open Time Machine.......You will have to set up everything again
Make sure Time Machine is "ON"
Click Select Disk
Choose the Time Capsule....named "Data" unless you have changed the name......as the destination for backups
Click Use Disk
Wait a few minutes, then click the Time Machine "clock" icon at the top of the screen
Click Backup Now
"Preparing" will take a long time....probably hours, even with an Ethernet connection. Let it run until the backup starts and completes or you receive an error message. If you try this using wireless, locate the laptop close to the Time Capsule, start the backup late at night and let it run overnight.....and it might be finished by the next morning.
It may appear as if Time Machine is making a complete new backup of your Mac......but after awhile, Time Machine should recognize the old backups and perform an incremental backup. However, it is also possible that Time Machine might make a new complete back up of your Mac, so let it complete if that is the case.