Modpack server

I am trying to download the rlcraft modpack, and at first got a message about forge/fml, but I did everything, downloaded the modpack on my computer and put it in my mods folder, and now it lists mods saying "requires version [insert number here] but mod is not found on client. I'm sorry, i've read other posts about this, but I am very dumb, so can someone just help me out here and explain to me what exactly I should do?

Like, do I do forge? what version should I be? I am clueless.

I tried that but it never opens. When I click play for rlcraft, it opens up the old launcher, and when I click play on the launcher, It used to just restart the launcher, but now the play button just says "loading..." forever. please help.

And now my server wont start. I click start and that little gear pops up for 2 seconds and then does nothing.