After the split x becomes (1 and z becomes 1). They can’t be converted to float. I think that’s why. Let me run the code. Edit - Also as you are returning a, b, c from splitter but Python functions return a single object. The a, b, c turns to a tuple. But you are using js syntax I think. So we have to unwrap inside the calculate function.

Now my original comment is not useful but you can try to introduce a conditional to check of the numbers x and z are actually numbers before converting to floats

# Ask user to enter an expression and display output

def main():
    expression = input("Expression: ")
    print(calculate (splitter (expression)))

# Split expression into components and assign to variables as float values

def splitter (expression):
    print(expression)
    x, y, z = expression.split()
    print(expression.split())
    return x, y, z

# Calculate expression result

# Changes Beginning
def calculate(numbers):
    x,y,z = numbers
# Changes End

    x, z = float(x), float(z)
    if y == "+":
        return str(round((x + z), 1))
    elif y == "-":
        return str(round((xz), 1))
    elif y == "*":
        return str(round((x*z), 1))
    else:
        return str(round((x / z), 1))

main()