I only know two reasons why you can't put an item on the market :
All players can accept an existing offer on the market, but only premium account players may submit offers. This means that, a free account player cannot initiate an offer - they can only browse the market for existing buy or sell offers from premium account players.
All offers (Buy and Sell) include a fee which ranges between 20gp and 100,000gp. This fee is 1% of the total price displayed on the offers and will be taken when the offer is created.
Source :TibiaFandom
I suspect you don't have enough gold in the bank to cover the item listing fee or you don't have a premium account.
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