Is your world 1.12.2 though? Recently with the 1.13 update mojang changed the way in which worlds were made, with new item ids which can’t be converted back to pre-1.13. If your world is on minecraft 1.13 or above you will need to use forge 1.13 or 1.14 in order to get mods on your world without losing all of your stuff. Before using the world with forge I also recommend backing it up in case it gets corrupted or some items and blocks go missing from your builds or the landscape. I reccomend forge 1.14. An easy way to to tell what Minecraft version you are using is to check the name of the minecraft window. It will say the version there, but if you still can’t find the version just check the ocean of your minecraft. If it has seaweed in the ocean you are on 1.13 or newer! There aren’t as many mods for 1.14 as for 1.12, but theres still a pretty cool selection. You also may be able to use the JEIDS mod to use your world in 1.12, but many items and blocks may go missing from your world.