I am new to Solidity and can't understand why I am getting the above error even though the function purchaseProduct() is declared as payable. I tried this code on Remix IDE and got this error.
Everything is working fine except for the purchaseProduct() function. I am trying to fetch the data from struct 'User', 'Product' and strore the data in a new struct that is 'Order'.
I have carried out validation using require(). While running the code on Remix:

1. Create 2 user
   
2. Create Product using first user and then,
   
3. purchaseProduct using second user

pragma solidity >=0.5.0; pragma experimental ABIEncoderV2; //Address: '0x7772B9117586b5F745bDb563a935E91b00449157' contract Buyproduct{ uint public orderCount = 0; mapping(uint => Order) public orders; uint public productCount = 0; mapping(uint => Product) public products; uint public userCount = 0; mapping(address => User) public users; struct Order { uint oid; address payable seller; address payable buyer; // string timestamp; string status; uint pid; string location; uint quantitiy; } struct Product { uint pid; string name; uint price; address payable seller; string info; uint quantity; } struct User { uint uid; string name; uint role; address payable user; string location; bool created; } event ProductCreated( uint pid, string name, uint price, address payable seller, string info, uint quantity ); event UserCreated( uint uid, string name, uint role, address payable user, string location, bool created ); event ProductPurchased( uint oid, address payable seller, address payable buyer, string status, uint pid, string location, uint quantity ); event ReviewAdded( uint rid, address reviewer, uint pid, string review ); function createUser(string memory _name, uint _role, string memory _location) public payable{ //No repeated address require(users[msg.sender].created == false, 'User already created'); //Increase userCount userCount++; //Add user users[msg.sender] = User(userCount, _name, _role, msg.sender, _location, true); //Trigger an event emit UserCreated(userCount, _name, _role, msg.sender, _location, true); } function createProduct(string memory _name, uint _price, string memory _info, uint _quantity) public payable { // Require a valid price require(_price > 0, 'Invalid Price'); // Increment product count productCount ++; // Create the product products[productCount] = Product(productCount, _name, _price, msg.sender, _info, _quantity); // Trigger an event emit ProductCreated(productCount, _name, _price, msg.sender, _info, _quantity); } function purchaseProduct(uint _id, /*string memory timestamp,*/ uint _quantity) public payable { // Fetch the product Product memory _product = products[_id]; // Fetch the owner address payable _seller = _product.seller; //Validate the buyer require(users[msg.sender].created == true, 'Unregistered user'); // Make sure the product has a valid id require(_product.pid > 0 && _product.pid <= productCount, 'Invalid Product ID'); // Require that there is enough Ether in the transaction require(msg.value >= _product.price, 'Not enough ether in Wallet'); // Require that the buyer is not the seller require(_seller != msg.sender,'Invalid Purchase'); //Incrmement orderCount orderCount++; // Transfer ownership to the buyer orders[orderCount] = Order(orderCount, _seller, msg.sender,/* ts,*/ 'Ordered', _product.pid, users[msg.sender].location, _quantity); //Reduce the quantity of the product _product.quantity -= _quantity; // Pay the contract by sending them Ether address payable wallet = address(uint160(address(this))); wallet.transfer(msg.value); // Trigger an event emit ProductPurchased(orderCount, _seller, msg.sender, /*timestamp,*/ 'Ordered', _product.pid, users[msg.sender].location, _quantity); } }'''