Is zakat obligatory on money in an investment portfolio locked for 15 years, knowing that breaking the portfolio before the end of the term leads to a loss of capital, profits, or both?

You are obligated to pay zakat (definition) on the amount along with its profit if it reaches the nisab (definition), and you must pay zakat on it every year. The fact that you cannot withdraw the amount except at a loss does not negate the obligation of zakat. The Islamic Fiqh Academy has affirmed the obligation of zakat on balances in investment accounts and their profits if they meet the conditions for zakat, whether they are long-term or short-term, even if no withdrawals have been made from them.