Hi ros281,
We understand you're unable to back up your computer. We're happy to help.
To get started, we'd suggest providing more specific details about the issue you have. What is happening when you navigate to System Preferences > Time Machine, and select the Backup option? Is it grayed out? Does the backup process start but just stays stuck in the same place and doesn't progress? Are you getting some sort of error? Etc.
Answers to questions like these will help give the community a better idea of the exact issue you're seeing so members can provide more relevant responses.
In the meantime, here's some basic steps for backing up with Time Machine that may be helpful: Back up your Mac with Time Machine
We look forward to your response. Cheers!