How can if a block under a player is water change it to ice?

I want to make a custom plugin for a friend, the idea is when a player walks over water, the water freezes but I get an error, please read and help me!
I seem to get an error using this code:

Code:

    public void changeBlocks() {
          for ( Player player : Bukkit.getOnlinePlayers()) {
              String playerName = player.getName();
              if (WaterFreezeUsers.contains(playerName)) {
                boolean isActive = WaterFreezeUsers.getPlayer(playerName);
                if (isActive) {
                    Location loc = player.getLocation();
                    loc.add(0, -1, 0);
                    Block block = loc.getBlock();
                    block.setType(Material.ICE);
                }
              }
          }
        }

As seen here the getPlayer(playerName) has an error and I do not know how to solve it, please help
Also if you could tell me how to change the block to ice IF the block beneath is water please say so too!

PD: Me nub at making plugins XD

 

You can use a PlayerMoveEvent to check the block that the player is currently on.

Code:

    @EventHandler
    public void onMove(PlayerMoveEvent e) {
  
        Player p = e.getPlayer();
  
        if(p.getLocation().getBlock().getRelative(BlockFace.DOWN, 1).getType().equals(Material.WATER) || 
        p.getLocation().getBlock().getRelative(BlockFace.DOWN, 1).getType().equals(Material.STATIONARY_WATER)){
      
            p.getLocation().getBlock().setType(Material.PACKED_ICE);
      
        }
    }

 

@EventHandler
public void walkWater(final PlayerMoveEvent e) {
Player p = e.getPlayer();
final Location loc = p.getLocation();
loc.setY(loc.getY() - 1);
loc.setX(loc.getX());
loc.setZ(loc.getZ());
final Material b = loc.getBlock().getType();
if (b == Material.WATER || b == Material.STATIONARY_WATER) {
loc.getBlock().setType(Material.PACKED_ICE);
}