I think it is null values as I suggested.
Try this amended query.
SELECT Total_x_mun.cvemun
, Total_x_mun.municipio
, Total_x_mun.COVID19
, Total_x_mun.SOSPECHOSO
, iif(Total_x_mun.total is null, 0, Total_x_mun.total)
, catmun.shape
, Switch(total_x_mun.total=0, 1,
total_x_mun.total<=10, 2,
total_x_mun.total<=50, 3,
total_x_mun.total<=100 , 4,
total_x_mun.total<=200, 5,
TRUE, 0) AS rango
, poblacion2015.poblacion
FROM (Total_x_mun
INNER JOIN catmun ON Total_x_mun.cvemun = catmun.cvemun)
INNER JOIN poblacion2015 ON catmun.cvemun = poblacion2015.mungem
ORDER BY Total_x_mun.total DESC;