Hello,
I am new to minecraft. I copy your .jar file into the mods folder in my '.minecraft' folder on my Linux machine
The other mods installed on my client are:

• journeymap-1.18.2-5.8.0beta17.jar
• vanillaplustools-1.18.2-1.0.jar
• forge-1.18.2-40.0.12-installer.jar

The server is running:

• paper-1.18.2-241.jar
• forge-1.18.2-40.0.12-installer.jar
• 4Gb of RAM for a max of 2 users

Thanks for your help in troubleshooting my issue

You executed the forge installer, right?

you cannot run paper and forge at the same time, only forge supports mods

Yes I run the jar file and it copied/downloaded the files
The journeymap mod is working just fine with forge