Hello, I'm using the Fabric version of the mod, when I run my game, I get the report that asks me to “Install forgeconfigapiport, version 3.1 or later.”. There's no info about where to get this, so I ask for help here. In my opinion, it shouldn't ask for a “Forge” Config when I play with Fabric…

It seems like that the fabric version of this mod needs https://www.curseforge.com/minecraft/mc-mods/forge-config-api-port-fabric as its front mod.

You are correct, Forge Config API Port is required to run the mod. This mod provides the configuration functionality that the mod uses, and it is named because it is a port of the configuration system used in Forge, which is much more feature-rich than the previously-used configuration system (Cloth Config). There is a link to it on the CurseForge mod page for Legendary Tooltips, or you can just use this link.