The reason you can’t join a trade will generally be that you already have an active trade and haven’t yet set your Steam WebAPI Token. Without a WebAPI Token, you are restricted to making just one skin purchase at a time, and you may only join one trade at a time from a user without a WebAPI Token. If the trade was listed by a user that does have a WebAPI Token, you may join multiple of these trades at a time.

Setting a WebAPI Token will ensure that your purchase has been recorded, which will make it easy to view every skin you have purchased and track which user has it. This will make it easier to join multiple trades should you choose to do so, without worrying about having any duplicate or invalid messages.

Once you set your WebAPI Token, these restrictions will all be lifted. You’ll be able to purchase and sell skins freely, allowing you to create and join as many trades at a time as you like, without seeing the invalid message.