Ensure your functions use the correct types, spelling, and capitalisation as below.
The variable collider can be renamed, and access modifiers are not important. The parameters are optional if you are not using them.
OnTriggerEntervoid OnTriggerEnter(Collider collider)
{
// Your behaviour here
}OnTriggerStayvoid OnTriggerStay(Collider collider)
{
// Your behaviour here
}OnTriggerExitvoid OnTriggerExit(Collider collider)
{
// Your behaviour here
}