Ensure your functions use the correct types, spelling, and capitalisation as below.
The variable collision can be renamed, and access modifiers are not important. The parameters are optional if you are not using them.
Unity, huh, how?
OnCollisionEnter2Dvoid OnCollisionEnter2D(Collision2D collision)
{
// Your behaviour here
}OnCollisionStay2Dvoid OnCollisionStay2D(Collision2D collision)
{
// Your behaviour here
}OnCollisionExit2Dvoid OnCollisionExit2D(Collision2D collision)
{
// Your behaviour here
}