ERC1155.sol

// contracts/GameItems.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

import "@openzeppelin/contracts/token/ERC1155/ERC1155.sol";
import "@openzeppelin/contracts/access/Ownable.sol";
import "@openzeppelin/contracts/utils/Strings.sol";

contract GameItems is ERC1155, Ownable {
    
    uint256 public constant CHARIZARD = 0;
    uint256 public constant IVYSAUR = 1;
    uint256 public constant VENUSAUR = 2;
    uint256 public constant CHARMANDER = 3;
    
    mapping (uint256 => string) private _uris;

    constructor() public ERC1155("https://bafybeihul6zsmbzyrgmjth3ynkmchepyvyhcwecn2yxc57ppqgpvr35zsq.ipfs.dweb.link/{id}.json") {
        _mint(msg.sender, CHARIZARD, 100, "");
        _mint(msg.sender, IVYSAUR, 100, "");
        _mint(msg.sender, VENUSAUR, 100, "");
        _mint(msg.sender, CHARMANDER, 100, "");
    }
    
    function uri(uint256 tokenId) override public view returns (string memory) {
        return(_uris[tokenId]);
    }
    
    function setTokenUri(uint256 tokenId, string memory uri) public onlyOwner {
        require(bytes(_uris[tokenId]).length == 0, "Cannot set uri twice"); 
        _uris[tokenId] = uri; 
    }
}

There might be issue metadata is not showing on opensea
https://testnets.opensea.io/collection/unidentified-contract-xfxdvbl2at

@itsanishjain you need to verify the contract aswell.