You could check for the presence of a decimal point in your string to decide if you want to coerce it into a float or an int.

number = input()

if '.' in number:
    number = float(number)
else:
    number = int(float(number))

By default, the data type of input is string. Pressing ENTER will return an empty string which cannot be casted into a float, since the string is empty and there is nothing to be casted, which is generating the error. Here are two solutions to handle this error.

Solution 1

Do not cast input into float directly, but cast it when assigning the value of input to any variable.

data=input("Enter a number or just ENTER to quit:")

Add the following statement before the loop to handle the condition when the user wants to exit without entering any number.

if not data: # If user wants to exit without entering any number
    Max = 0
    Min = 0
else:  # Cast the inputs by user to float
    Max = float(data)
    Min = float(data)

Lastly add the following line at the beginning inside the loop.

data = float(data)

Complete Code

count=0.0
Sum=0.0
average=0.0
data=input("Enter a number or just ENTER to quit:")
if not data:
    Max = 0
    Min = 0
else:
    Max = float(data)
    Min = float(data)

while data!='':
    data = float(data)
    count+=1 
    number=float(data)
    Sum+=number
    average=Sum/count

    if data<Min:
        Min=data
    if data>Max:
        Max=data
    data=input("Enter a number or just ENTER to quit:")

print(count,("numbers entered."))
print("Sum:",Sum)
print("Average:",average)
print("Min:",Min)
print("Max:",Max)

Solution 2

What you can also do is set a default value for empty inputs. Here since you are casting input into a float, you can set any number as the default value for empty inputs. However, I will not prefer this because if the user enters the same number as the default value, the loop will be terminated. In the following example I've only modified two lines of your code and set the default value for empty inputs as 0.

count=0.0
Sum=0.0
average=0.0
data=float(input("Enter a number or just ENTER to quit:") or 0)
Min=data
Max=data

while data!=0:
    count+=1
    number=float(data)
    Sum+=number
    average=Sum/count

    if data<Min:
        Min=data
    if data>Max:
        Max=data
    data=float(input("Enter a number or just ENTER to quit:") or 0)

print(count,("numbers entered."))
print("Sum:",Sum)
print("Average:",average)
print("Min:",Min)
print("Max:",Max)