I am using ubuntu $18\mathrm{.}04,$ python $3\mathrm{.}6\mathrm{.}9$When i run "sudo get $-$ update" it says error $403$ forbidden $[$shows an IP address$]$

Failed to fetch $[$shows a link...bionic security Inrelease is not signed$]$ Updating from such a repository cannot be done securely and therefore disabled by defaultSee apt $-$ secure$(8)$ manpage for repository creation and user configuration details$($ This is just a snippet of the error since i cannot post full error here$)$p$.$s my wifi works fine and no firewall on my system.