WiFi working But Time Capsule cannot backup

No ethernet connection to desktop which is Mac Pro

This is more serious than the backup failure which is fairly common.

Is the ethernet connectivity light on or off? (tiny green led on the port itself).

What is the model of the TC? A1xxx from the base please.. or if you know it Generation.. which you can also pull from the airport utility.

Model number or part number are much easier to work with.. apple has the tiniest print in the history of legal contracts I think.. plus poor contrasting colors so as to make their labels impossible to read.. but I think yours if the tall one.

What OS is the computer running?

This is stock standard Yosemite problem.. did you reboot the TC.. and if that fails do a full restart of the network in correct order??

Boeing aircraft and every PC ever made need this .. Macs too nowadays. As well as peripherals for them.

http://www.engadget.com/2015/05/01/boeing-787-dreamliner-software-bug/

Doesn't it make you feel safe knowing the aircraft you're in needs a reboot occasionally to keep power turned on.

Have you checked the TM log to find out what the problem is?? Or what error message do you get?

Have you tried accessing the TC from finder in the Mac Pro??