When Sessions was AG he decided to go after states that legalized marijuana, Trump shut him down immediately. Anyone seeing raids on dispensaries? Federal government shutting down commercial growers licensed in states which have legalized weed? I haven't.
The former Michigan AG that ran for governor in 2018 spent years harassing medical marijuana caregivers and sending them to prison, he opposed legalization and also opposed Trump being elected. During a rally in Michigan that AG showed up for Trump to endorse him. Trump did so by telling the crowd Schuette refused to endorse Trump in 2016, that he was a Bush, then Rubio, then anybody but Trump supporter. Schuette, who opposed legalization in the state lost the election.
That is what Trump meant by not wanting Republicans to put marijuana legalization on their state ballots in 2020.