Fingerprint and arduino nano

Hello I have fingerprint sensor R305 from adafruit .and am using arduino nano every thing good but wen I enroll it says
Place the finger
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Remove finger
Place the finger again
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UNKNOW ERROR .
I don't know why am getting unknow error
Am using the new version of arduino 1.8
I read a lot about this issue and they said that it will work with arduino 1.6.6
And I don't know how to get this old version to download where I can find it ?
And why it won't work with the new version
Thanks .
I appreciate the help

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