Why do I get error when trying to copy dollar bills

The Secret Service gives some info (i.e. you can copy money is it's smaller than regular currency and one-sided. If it's just for a prop in a skit use real money or if you need to use a ton of it just use cut up green paper or actual monopoly money:

KNOW YOUR MONEY

Illustrations of Currency, Checks or Other Obligations

The law sharply restricts photographs or other printed reproductions of paper currency, checks, bonds, revenue stamps and securities of the United States and foreign governments.

For additional illustrations of U.S. currency visit the Bureau of Engraving and Printing.

U.S. Currency

The Counterfeit Detection Act of 1992, Public Law 102-550, in Section 411 of Title 31 of the Code of Federal Regulations, permits color illustrations of U.S. currency provided:

• The illustration is of a size less than three-fourths or more than one and one-half, in linear dimension, of each part of the item illustrated

• The illustration is one-sided

• All negatives, plates, positives, digitized storage medium, graphic files, magnetic medium, optical storage devices, and any other thing used in the making of the illustration that contain an image of the illustration or any part thereof are destroyed and/or deleted or erased after their final use