how to call amount in the method

You have to bind amount to self, i.e.

amount = StringVar()

should be

self.amount = StringVar()

Accordingly, you also have to change my_entry = Entry(frm_upper, textvariable=amount) to my_entry = Entry(frm_upper, textvariable=self.amount)

Otherwise, self.amount is not defined when you do bill = float(self.amount.get()) in calculate_10_perc.

Also, note that doing

total_result = Label(...).grid(row=2, column=1)

total_result is not the Label but the result of grid, i.e. None

It seems like you are (trying to) create a new label each time you update the total value. Instead, you should configure the existing label to hold the new value. in __init__:

self.total_label = Label(frm_mid, text="")self.total_label.pack()

And in calculate_10_perc:

self.total_label.configure(text="Total after tip added: %.2f" % total)

Finally, given that all those calculate_X_perc functions will probably look very similar, you could make it one function and pass the percent as a parameter, e.g. in __init__:

btn_10_perc = Button(frm_lower, text = "10%", command=lambda: self.calculate_perc(1.1))

And the function:

def calculate_perc(self, percent):    bill = float(self.amount.get())    self.total_label.configure(text="Total after tip added: %.2f" % (bill * percent))

Ok, so the issue here is that you're not making amount an instance variable in __init__:

class TipCalc:    def __init__(self):        my_window = Tk() # create a window        my_window.title("Tip Calculator")        my_window.geometry('400x200')        amount = StringVar()        ...

Notice that you're simply declaring amount, not self.amount. This means that amount is a normal variable in __init__ that disappears when __init__ finishes, not an instance variable that lives as long as the instance exists.

Then you're getting your error here:

def calculate_10_perc(self):    bill = float(self.amount.get())

Because self.amount isn't a thing at this point - amount disappeared when __init__ finished.

So probably the easiest solution is to simply change __init__ and make amount an instance variable:

class TipCalc:    def __init__(self):        my_window = Tk() # create a window        my_window.title("Tip Calculator")        my_window.geometry('400x200')        self.amount = StringVar()        ...

Then, be sure to replace all the references to amount with self.amount.