Message after installing forge (forge mod loader has found a problem with your Minecraft installation)?

I wanted to install forge for shematica today. Everything worked. The order "mods" with LunatriusCore and the things were all ready and forge was also there when selecting the Minecraft versions. However when starting Minecraft this message came:

What else can I do to install Shematica?

I'd be happy for help.

That the current version is not supported.

What can I do about it?

Use 1.8.9

I have

Should I then use the latest version of forge? So 1.15 or whatever that is.

You need to download a version of Lunatrius Core that is compatible with 1.8. The version used by is only compatible with 1.8.9

Thank you but how do I know that the version is then compatible with 1.8?

https://www.curseforge.com/...es/2269629

by looking for which version the developer has provided the mod on the corresponding website.

Thank you.