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.
OnCollisionEntervoid OnCollisionEnter(Collision collision)
{
// Your behaviour here
}OnCollisionStayvoid OnCollisionStay(Collision collision)
{
// Your behaviour here
}OnCollisionExitvoid OnCollisionExit(Collision collision)
{
// Your behaviour here
}