You have somehow copied the hyperlink meta-data when you added the FeatureExtractor option. It should look like this:
SynthesizeMissingValues[{1, Missing[]}, FeatureExtractor -> "Minimal"]
Although, I'm sceptical that that's what you're really wanting to do. One data point isn't enough to meaningfully generate missing values. It'll produce an output, but I don't know what it's worth. Are you trying to use the learned distribution to generate values? If so, then you need to add the distribution, something like this:
ld = LearnDistribution[{{2, 1.4}, {3, 3.1}, {5, 5.4}, {7, 7.5}}];
SynthesizeMissingValues[ld, {1, Missing[]},
FeatureExtractor -> "Minimal"]
