I think the compiler errors are fairly self-explanatory, but here's a fixed version:

```solidity
pragma solidity ^0.5.0;

contract Courses {
    struct Instructor {
        uint age;
        string fName;
        string lName;
    }
    
    mapping (address => Instructor) instructors;
    address[] public instructorAccts;
    
    function setInstructor(address _address, uint _age, string memory _fName, string memory _lName) public {
        Instructor storage instructor = instructors[_address];
    
        instructor.age = _age;
        instructor.fName = _fName;
        instructor.lName = _lName;
    
        instructorAccts.push(_address);
    }
    
    function getInstructors() public view returns (address[] memory) {
        return instructorAccts;
    }
}
```

* I gave a proper type to `instructor`.
* I gave a data location for `instructor`, though it was really unclear to me whether that should be `storage` or `memory`. If `storage`, it's going to duplicate each instructor (change their details and then also push a new copy onto the end of the array). If `memory`, it's reading from `instructors` for no reason. Neither makes much sense to me. :-)
* I gave the return value of `getInstructors` a data location.
* I deleted the strange `-1` that wasn't doing anything.