Its a network error.
You need to make the network a bit more orientated to the Time Capsule.
What we need is full info on your setup.
What is your main modem router.. make and model please??
How is the TC setup and what model is it?
Screenshots from airport utility help loads.
Also these problems multiplied at High Sierra.. when Apple increase network security. What OS is your computer running.
How do you connect from this mac to the network?
wireless or ethernet.. direct to the TC or via the main router??
All these things mean there is not a single simple answer but usually we can find something to help.
However you should note.. The reliability of time machine is really an issue in these later OS.. from el capo on but worse in each succeeding version. I gave up.. went to Carbon Copy Cloner which is just hugely superior from a network and reliability point of view.. even if it is not so easy to use.. at least it has never decided to wipe out its own backup which TM did regularly.