Single Jump w Layer Mask

 private Rigidbody playerRB;

 public float jumpVelocity = 10;
 private bool isJumping;

 public float distanceToGround = 0.1f;
 public LayerMask groundLayer;
 private CapsuleCollider playerCollider;

 void Start()
 {
     playerRB = GetComponent<Rigidbody>();
     playerCollider = GetComponent<CapsuleCollider>();
 }

 void Update() {

     isJumping |= Input.GetKeyDown(KeyCode.J);
 }

 private void FixedUpdate() {

     if (IsGrounded() && isJumping)
     {
         playerRB.AddForce(Vector3.up * jumpVelocity, ForceMode.Impulse);
     }
     isJumping = false;
 }

 private bool IsGrounded() {

     Vector3 capsuleBottom = new Vector3(playerCollider.bounds.center.x, playerCollider.bounds.min.y, playerCollider.bounds.center.z);
     bool grounded = Physics.CheckCapsule(playerCollider.bounds.center, capsuleBottom, distanceToGround, groundLayer, QueryTriggerInteraction.Ignore);
     return grounded;
 }