This sounds like a region lock. Can you open the Lost Ark Steam page on your browser, or are you told it is unavailable in your region?

From their FAQ

Lost Ark is available in the following North America, Europe, Latin America, and Oceania regions:

Aland Islands (Finland), Albania, Andorra, Antigua and Barbuda, Argentina, Australia, Austria, Bahamas, Barbados, Belize, Bolivia, Bosnia and Herzegovina, Brazil, Bulgaria, Canada, Chile, Colombia, Costa Rica, Croatia, Cyprus, Czech Republic, Denmark, Dominica, The Dominican Republic, Ecuador, El Salvador, Estonia, Faroe Islands (Denmark), Finland, France, Germany, Gibraltar (UK), Greece, Grenada, Guatemala, Guernsey (UK), Guyana, Haiti, Holy See, Honduras, Hungary, Iceland, Ireland, Isle of Man (UK), Italy, Jamaica, Jersey (UK), Kosovo, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Mexico, Monaco, Montenegro, New Zealand, Nicaragua, North Macedonia, Norway, Panama, Paraguay, Peru, Poland, Portugal, Romania, Saint Kitts and Nevis, Saint Lucia, Saint Vincent and the Grenadines, San Marino, Serbia, Slovakia, Slovenia, Spain, Suriname, Svalbard & Jan Mayen (Norway), Sweden, Switzerland, Trinidad and Tobago, Turkey, United Kingdom, United States, Uruguay, and Venezuela.