compilation - Sub line gives compile error: variable not defined -

upon trying run code error "compile error: variable not defined" , highlights sub line. here code reference.

sub ri_processing()     dim single     dim output() single 'define dynamic array solver output  process dmso data     = 1 45 step 1         workbooks("calibration range 1 normalized profiles.xlsx").activate         rawdata.range("c10:c649") = sheet1.columns("i").values         solverok setcell:="$e$7", maxminval:=2, valueof:=0, bychange:="$b$2:$b$4", _         engine:=1, enginedesc:="grg nonlinear"         solversolve         workbooks("simulated fresnel function.xlsx").activate         output(i, 1) = sheet1.range("b4").values      next      = 0      ' process nacl data     = 1 102 step 1         workbooks("calibration range 1 normalized profiles.xlsx").activate         rawdata.range("c10:c649") = sheet2.columns("i").values         solverok setcell:="$e$7", maxminval:=2, valueof:=0, bychange:="$b$2:$b$4", _         engine:=1, enginedesc:="grg nonlinear"         solversolve         workbooks("simulated fresnel function.xlsx").activate         output(i, 2) = sheet1.range("b4").values      next      = 0      ' process sucrose data     = 1 72 step 1         workbooks("calibration range 1 normalized profiles.xlsx").activate         rawdata.range("c10:c649") = sheet3.columns("i").values 'copies data of 'i' columns raw data solver         solverok setcell:="$e$7", maxminval:=2, valueof:=0, bychange:="$b$2:$b$4", _         engine:=1, enginedesc:="grg nonlinear"         solversolve 'run solver         workbooks("simulated fresnel function.xlsx").activate         output(i, 3) = sheet1.range("b4").values 'put solver output matrix 'output' in appropriate columns      next  end sub