My NLT Bible is missing Acts 8:37?

It is missing from most modern translations. In short the manuscript evidence is very weak for the inclusion of this verse. There are around 6000 old handwritten New Testaments and New Testament parts at our disposal. These range back into the 3rd century with a couple fragments going earlier to the advent of the printing press. There are some variations among them. And you have stumbled across one. For details I suggest you google this and do a little more research.