I want to know how I can get just liquid with no sugar crystals added
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It's possible that you're over-reducing and there isn't enough liquid to hold the sugar, but if you're following the recipe that seems unlikely. This: http://www.southendformaggio.com/recipe-for-sweetened-condensed-milk/ says that stirring can induce crystallization, so try just stirring to combine and then leaving it be until it's done.
SMSF is a trusted home cook.
The current issue of Fine Cooking has an article and recipe on making sweetened condensed milk - maybe this will be helpful to you: