F# If statement expecting unit from float

the statement is fine until the final elif which it then has the error "Was expected to have type unit"

type BankAcc = {AccNum:int; mutable  Balance:float} with

    member this.Withdraw(amount,?withdrawal) =
     let withdrawAmount = this.Balance * float amount
     match withdrawal with
      | None -> withdrawAmount
      | Some deduct -> withdrawAmount - deduct
let Account ={AccNum=123;Balance = 15.00}

Account.Withdraw(25.00) // withdrawing 25 from an account with a balance of 15

let test Balance withdrawAmount =
  if Balance = withdrawAmount then "Equals"
  elif Balance < withdrawAmount then "Balance too low"
  else Balance - withdrawAmount 

  Account={AccNum =0001; Balance=0.00};   

let CheckAccount Balance = 
    if Balance < 10.00 then "Balance is low"
    elif Balance >= 10.00 && Balance <= 100.00 then "Balance is ok"
    elif Balance > 100.00 then "balance is high"

let sort = Seq.unfold(fun Balance -> if(snd Balance => 50)then List.map(fun accounts-> Balance <50) list1)