Using https://mirrors.jenkins.io/debian/jenkins_2.426_all.deb results in a redirect to https://2.mirrors.in.sahilister.net/jenkins/debian/jenkins_2.426_all.deb, which on GET returns a 404
The Jenkins mirrors have a subset of Jenkins releases. Your request is asking for a weekly release that is over a year old. That weekly release is not available on the mirrors. That release also has multiple critical security vulnerabilities.
You should install a newer version following the instructions for Linux installation:
If you absolutely must have that specific version, it is still available on some of the mirrors, like:
It is also available from the Jenkins archives at: