Looks like there is a problem in one of the mods trying to use a method that doesn’t exists in your version of Forge, either the mods is built using and older version of Forge and has it’s mappings wrong, or it’s using a newer version and you’re using an older version, contact the mod author, of the I’m guessing, Flansmod, that’s the name in the error: com.flansmod.client, contact them for help.