Fix ValueError: could not convert string to float:

You are immediately trying to cast your input to a float, which fails when something other than a number is entered. You could fix this by checking for a blank input before doing the cast:

...
count = input('Please enter another number: ')
if count == (''):
    break
count = float(count)
...

You do have a number of other logical errors in your code at this point, but that should at least get you past the value error.

You can try just using a try and catch statement to handle a possible ValueError:

try:
  count = float(input('Please enter another number: '))
except ValueError:
  break

However, I do think it's worth noting that there are other parts of your code that are incorrect, so if you could give a bit more depth into what exactly you are trying to code, we can help you fix other parts if you need it.

Simply you can check weather the input is a number or not

value = (input('Please enter number: '))
total = 0
sum = 0
while value.isnumeric():
    second_input = input('Please enter another number: ')

    if not second_input.isnumeric():
         break

    count = float(second_input)
    sum =+ (1)
    total = count + count
   
average = (total+value/sum)
print(float(average))

Working code with exception handling:

value = ""
while True:
    try:
        value = float(input('Please enter number: '))
        break
    except:
        print("Invalid number")

total = 0
sum = 1 # It is 1 if including the first value, can be change to 0 if isn't
while value == value:
    try:
        count = input('Please enter another number: ')
        count = float(count)
        sum =+ (1)
        total = total + count # Changed this line as OP asking for average
    except ValueError:
        if count == (''):
            break
        else:
            print("Invalid number, press enter to exit loop.")
average = (total+value/sum)
print(float(average))