The reason for this error is that the format specifier d is specifically for formatting integers, not floats.You can use the general format specifier f instead.

f'{int(1223.555):5d}'

f"..." expressions format values to strings, integrate the result into a larger string and return that result. That's not quite the same as 'casting'^*.

number is an expression here, one that happens to produce an integer object. The integer is then formatted to a string, by calling the __format__ method on that object, with the first argument, a string containing a format specifier, left empty:

>>> number = 10
>>> number.__format__('')
'10'

We'll get to the format specifier later.

The original integer object, 10, didn't change here, it remains an integer, and .__format__() just returned a new object, a string:

>>> f"{number} is not a string"
'10 is not a string'
>>> number
10
>>> type(number)
int

There are more options available to influence the output by adding a format specifier to the {...} placeholder, after a :. You can see what exact options you have by reading the Format Specification Mini Language documentation; this documents what the Python standard types might expect as format specifiers.

For example, you could specify that the number should be left-aligned in a string that's at least 5 characters wide, with the specifier <5:

>>> f"Align the number between brackets: [{number:<5}]"
'Align the number between brackets: [10   ]'

The {number:<5} part tells Python to take the output of number.__format__("<5") and use that as the string value to combine with the rest of the string, in between the [ and ] parts.

This still doesn't change what the number variable references, however.

^*: In technical terms, casting means changing the type of a variable. This is not something you do in Python because Python variables don't have a type. Python variables are references to objects, and in Python it is those objects that have a type. You can change what a variable references, e.g. number = str(number) would replace the integer with a string object, but that's not casting either.

Include the type specifier in your format expression:

>>> a = 10.1234
>>> f'{a:.2f}'
'10.12'