Can't find the problem in the string fuction

So the problem was this

error: invalid operands of types ‘const char [20]’ and ‘float’ to binary ‘operator<<’

that came out on my string fuction. I tried searching up in google but nothing came up.

#include <iostream>
#include <string>
using namespace std;

class Rectangle
{
    public:
    Rectangle () : length(1.0), width(1.0)
    {
        
    }
    Rectangle(float l, float w) :length(l), width(w)
    {
    }
    void setLength(float l)
    {
        length = l;
    }
    float getLength()
    {
        return length;
    }
    void setWidth(float w)
    {
        width = w;
    }
    float getWidth()
    {
        return width;
    }
    double getArea()
    {
        return width * length;
    }
    double getPerimeter()
    {
        return 2 * (width + length);
    }
    string toString()
    {
        string s1 = "Rectangle [Length ="<<getLength()<<", width = "<<getWidth()<<"]";
        return s1;
    }
    
    private:
    float length = 1.0;
    float width = 1.0;

};

Here's my code. So what is the problem in my string fuction? PS. also, it would be great if you point out the other problems that I missed.

You're probably mixing printing to an output stream via << with concatenating strings.

To concatenate 2 std::strings, you use the + operator instead. The operator also can work with things similar to a string like char const*, but only if one of the operands already is a std::string.

Use std::to_string for converting the numbers to string representation:

std::string toString()
{
    std::string s1 = "Rectangle [Length =" + std::to_string(getLength()) + ", width = " + std::to_string(getWidth()) + "]";
    return s1;
}

Alternatively use std::ostringstream to create the string allowing you to use the << operator:

#include <sstream>
#include <utility>

...

std::string toString()
{
    std::ostringstream stream;
    stream << "Rectangle [Length =" << getLength() << ", width = " << getWidth() << "]";
    return std::move(stream).str();

   // note: str on a rvalue reference may be cheaper starting C++20
   //       in prior versions the line above equivalent to
   // return stream.str();
}