Read the full message
Write down the mod that requires Placebo and the allowed dependency range. Missing can mean no jar was discovered, but a message can also describe an installed version that is too old or incompatible. The exact wording determines your next step.
Check the instance folder
Open the game directory belonging to the profile you launched, then inspect mods. A jar in another profile or only on your desktop cannot satisfy this instance. Confirm the downloaded file is the normal mod jar for the same Minecraft release and loader.
Restart with one compatible file
Close the game, replace any incompatible Placebo jar and launch again. If the error persists, inspect latest.log for the first relevant dependency message. Keep a record of what changed instead of adding several Placebo versions at once, which can create a duplicate-mod error.