My new job had a finger print back ground check. It showed year, arrest, and robbery in that order. Nothing else. It never happened. How do I get it erased from records.

First a fingerprint check merely tells your ID and if you used other names when you were arrested or when you were fingerprinted for other reasons.

A NCIC check then takes those names you used or show up in the fingerprint check against the record of arrests.

Since it is showing an arrest and conviction it means that they would have had someone with the same fingerprint ( not possible) or that they got the system confused.
OR this arrest was done without fingerprints and they are matching name and info .

Many people when arrested use the ID of others but their fingerprints will not match.

This is going to take some time to clear up and will require an attorney ( and may be costly)

Document every little thing you are told and sent and when, in case you can sue.
See if you can get where and when this happened etc from the employer.
Double check the full name, DOB, SSN they used on you. One letter or digit off could do it!
Visit your police station and ask if they can help clear you.
Do a background criminal check on yourself for about $35 - 40. Google various sites.