The phrase was and is not intended to express incapacity but, on the contrary, absolute moral determination to obey the Catholic Faith.
“In the 19th and 20th centuries, non possumus dominated the diplomacy of Popes Pius IX, Leo XIII, St. Pius X, Benedict XV, and Pius XI, especially after the capture of Rome when the Supreme Pontiff became the prisoner in the Vatican and chose to limit his contact with the outside world.”
Here is an article that gives the circumstances of Seifert’s dismissal: Josef Seifert, OnePeterFive – May 10, 2021