This is an old machlokes. The Rema paskens you don’t ask the dead person to do anything for you, and the gemera in Sota (34?) about Kaleiv asking the avos to pray for him is agaddateda.
The Pri Megadim defends it and says it’s not a violation of the 5th ikkar because they are asking him to intercede with Hashem. The dead person isn’t all-powerful and isn’t Hashem in a human body.
The rebbe’s atzmus sicha said you can directly daven to a rebbe because he’s Hashem in a human body. That is NOT what the pri megadim said. That is the person in the kever as a god.
Major difference.
Though litvaks hold like the Rema and do neither. We see how innocent practices become avoda zara r”l