command line – Error getting public keys

When I execute some apt command like install or update, it produces multiple errors of public keys not found and it can’t finish executing the install or update. For example, this is one of the multiple same errors (my console is in Spanish):

Se produjo un error durante la verificación de las firmas. El repositorio no está actualizado y se utilizarán los ficheros de índice antiguos. Error de GPG: https://mega.nz/linux/MEGAsync/xUbuntu_20.04 ./ InRelease: Las firmas siguientes no se pudieron verificar porque su clave pública no está disponible: NO_PUBKEY 03C3AD3A7F068E5D

I’ve already tried with

sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys (key)

for each key, but every key gives me one of two errors:

"gpg: ADVERTENCIA: no se ha proporcionado ninguna orden. Intentando adivinar lo que quieres...
uso: gpg (opciones) (filename)"

, which in English is something like

"gpg: WARNING: no order has been provided. Trying to guess what you want..."

; and

"gpg: recepción del servidor de claves fallida: Sin nombre" 

if I execute

gpg --keyserver wwwkeys.eu.pgp.net --recv-keys (key)

I also can’t install PPA Manager because every tutorial implies executing

sudo apt-get update
sudo apt-get install

which gives the errors. I also tried executing

sudo rm /etc/apt/trusted.gpg

before getting the keys.

I’ve also tried

PUBKEY=(key)
gpg --keyserver subkeys.pgp.net --recv-key $PUBKEY && gpg -a --export $PUBKEY | 
sudo apt-key add --

And I get

“gpg: recepción del servidor de claves fallida: Expiró el tiempo de
conexión”

I have Ubuntu 20.