Áñez came to the presidency in the middle of protests where the president-elect (Evo Morales), the vicepresident (Álvaro García Linera) and senate president (Adriana Salvatierra) have to resign.
First point here: Can a forced resignation by the army/oposition be legitimized? If your president-elect has to leave the country under pressure from the opposition, isn't that a form of coercion and ultimately terrorism?
After that it seems Áñez was next in the constitutional line of succession (according to wikipedia)
The role of acting president of the Senate would normally fall to the vice president of the Senate, Rubén Medinaceli, but since he also had resigned, the position fell to Añez as second vice president.
Also, some countries recognized her as president.
After Áñez took office, the governments of Canada, Brazil, the European Union, Russia and the United States recognized her as the acting president of Bolivia.
But now is under arrest with charges of terrorism and sedition.
Bolivian Attorney General's Office issued an arrest warrant for Jeanine Áñez and five members of her government for terrorism, sedition, and conspiracy on 12 March 2021.
It is a contradiction to be the legitimate president and at the same time to be accused of sedition. So my question is, was Jeanine Áñez really a legitimate president?