Comparing Squareroots error

I am trying to compare the values of the variable 'x' to the squareroot of another variable

right now it says 'attempt to call a nil value' or some variation

here is my code.

i dont know what to do

sticks = 5
woodenTools = 10
scrap = 15
woodBlocks = 15
mushroomBlocks = 15
woodenScaffolding = 15
coal = 80
charcoal = 80
peat = 80
blazeRod = 120
coalCoke = 320
biofuelCan = 520
lava = 1000
coalfuelCan = 1520
--if square root of lenth * width is less than or greater than something, give that as recommended fuel.
if x <= sqrt(sticks) then
  recomendedFuel = "Sticks"
  fuel = sticks
end
if sqrt(sticks) <= x <= sqrt(woodenTools) then
  recomendedFuel = "Wooden Tools"
  fuel = woodenTools
end
if sqrt(woodenTools) <= x <= sqrt(scrap) then
  recomendedFuel = "Scrap, Wood Blocks, Mushroom Blocks, Wooden Scaffolding"
  fuel = scrap
end
if sqrt(scrap) <= x <= sqrt(coal) then
  recomendedFuel = "Coal/ Charcoal, Peat"
  fuel = coal
end
if sqrt(coal) <= x <= sqrt(blazeRod) then
  recomendedFuel = "Blaze Rod"
  fuel = blazeRod
end
if sqrt(blazeRod) <= x <= sqrt(coalCoke) then
  recomendedFuel = "Coal Coke"
  fuel = coalCoke
end
if sqrt(coalCoke) <= x <= sqrt(biofuelCan) then
  recomendedFuel = "Biofuel Can"
  fuel = biofuelCan
end
if sqrt(biofuelCan) <= x <= sqrt(lava) then
  recomendedFuel = "Lava"
  fuel = lava
end
if sqrt(lava) <= x <= sqrt(coalfuelCan) then
  recomendedFuel = "Coalfuel Can"
  fuel = coalfuelCan
end

math.sqrt not sqrt