1987 190e Mercedes the ignition switch the key will not turn it is the same key i bought the car with a month ago?

If the key is the cut Blade type key, it is worn.... try the spare key. If the newer key does not turn then the lock cylinder is worn... get it replaced ASAP before you get stuck somewhere.

Also try rocking the steering wheel while turning the key... maybe the steering wheel lock is loading the release pin making it hard to turn the key.

Never put the blade type key on a big keyring with a dozen other keys hanging from it!!! The ignition lock cylinder will be damaged resulting in the key not turning.

Try a lock lubricant in the lock cylinder to help the lock mechanism turning.

Good Luck...