The election on October 20th was rigged
The Bolivian constitution, approved under Morales' government in 2009, established a two-term limit to the presidency.
Evo Morales' run for re-election for his third term in the 2014 elections stood against constitutional term limits. Moreover, in February 2016 Morales put forward a referendum, asking Bolivians to abolish presidential term limits, therefore allowing him to run for a fourth term. The president lost the referendum in a split vote (51.3% against 48.7%) yet still chose to run for a fourth term in October 2019.  Even if the election had been conducted legitimately, Morales was still an illegitimate candidate.
Evo Morales has defended his right to run for the presidency on both occasions. The constitution approved in 2009 established a new state: the Plurinational State of Bolivia. When Morales run in 2014, it was the first time he was running to become the president of the newly-created state. In regards to the 2016 referendum, Morales argued that it was his human right to be an elected official. In both cases, Morales presented the case to the Supreme Court, which ruled in his favor both in regards to the 2014 and 2016 cases. 
1. The Bolivian constitution establishes a two-term limit for presidents. 2. Term limits had been reiterated in a popular referendum, which Morales lost in 2016. 3. Evo Morales was running for his fourth presidential term. 4. Evo Morales' candidacy was therefore unconstitutional.
1. In 2014, Evo Morales was running on his first term as president of the newly-constituted Plurinational State of Bolivia. 2. The Supreme Court further established that it was Morales' fundamental right to become an elected representative of the people. 3. Morales had, therefore, the right to his own candidacy under Bolivian law.