Broken packages

Second attempt as first disappeared
I have updated my old Dell i386 with a newer version of Debian however have lost Linuxcnc and have spent countless hours trying to reinstate. I copy line by line and paste to ensure no errors. When I get to "sudo apt install gdebi linux-headers-4.19.0-8-common-rt linux-headers-4.19.0-8-rt linux-image-4.19.0-8-rt-amd64" this results in :
Unable to locate package linux-image-4.19.0-8-rt-amd64
E: Couldn't find any package by glob 'linux-image-4.19.0-8-rt-amd64'
E: Couldn't find any package by regex 'linux-image-4.19.0-8-rt-amd64'
Using Synaptic I cant find this file. What am I doing wrong
I have tried possible incorrectly downloading an alternative but they always come with errors
Any help very much appreciated

sorry Broken

Thanks for quick reply
I have spent hours on this page following instructions line by line but still end up with missing image.
How can I find the missing image?

I have tried Method 2, 2nd line
"E: Unable to locate package linux-image-4.19.0-9-rt-amd64
E: Couldn't find any package by glob 'linux-image-4.19.0-9-rt-amd64'
E: Couldn't find any package by regex 'linux-image-4.19.0-9-rt-amd64'"
I cant find this file

Ign:1 cdrom://[Debian GNU/Linux 10.5.0 _Buster_ - Official i386 NETINST 20200801-13:11] buster InRelease
Err:2 cdrom://[Debian GNU/Linux 10.5.0 _Buster_ - Official i386 NETINST 20200801-13:11] buster Release
Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
Hit:3 deb.debian.org/debian buster InRelease
Hit:4 deb.debian.org/debian buster-updates InRelease
Hit:5 security.debian.org/debian-security buster/updates InRelease
Hit:6 buildbot.linuxcnc.org buster InRelease
Reading package lists... Done
E: The repository 'cdrom://[Debian GNU/Linux 10.5.0 _Buster_ - Official i386 NETINST 20200801-13:11] buster Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: Target Packages (main/binary-i386/Packages) is configured multiple times in /etc/apt/sources.list:3 and /etc/apt/sources.list:5
W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:3 and /etc/apt/sources.list:5
W: Target Translations (main/i18n/Translation-en_GB) is configured multiple times in /etc/apt/sources.list:3 and /etc/apt/sources.list:5
W: Target Translations (main/i18n/Translation-en) is configured multiple times in /etc/apt/sources.list:3 and /etc/apt/sources.list:5

apt-get upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following packages were automatically installed and are no longer required:
python-serial python-yapps
Use 'sudo apt autoremove' to remove them.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

Looks like there are problems??? How do I overcome?

partner and source code repositories Have taken out cdrom stuff but a bit lost with this