this should work for what you want: var map = thisComp.layer("Luma_Effector");
var pos = map.fromWorld(toWorld(anchorPoint));
var sample = map.sampleImage([pos, pos], [1, 1], true);
var mapValue = linear((sample + sample + sample) / 3, map.effect("Scale All Low Value")("Slider"), map.effect("Scale All High Value")("Slider"));
Explanation: Since scale needs 2 values (x and y) the value is stored in an array. You can recognize arrays by the "[" and "]" brackets and the different values are seperated by commas. To get a certain value from an array you reference its position again with square brackets and the corresponding number. Arrays always start at 0 so the first value is always something like "example". So with this in mind, I replaced the first part of your array with "value", which represents the entered x value, which is still keyframeable.