Check to see if the RD J-wheels are not hitting/binding on the freewheel... if so pull the RD arm back to allow the chain to engage the sprocket and the dropouts.
If not as others have said go back and check the spacing as well as the opening on the quick release.
Good luck
JM2C's
Ben
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"EVERY PERSON IS GUILTY OF ALL THE GOOD THEY DID NOT DO"
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