Discord.js cannot read property

This is pretty easy to solve. The problem is that voiceStateUpdate does indeed take two variables, however they are not oldMember, newMember but oldState, newState.

As usual with functions it doesn’t really matter what you call them but it makes more sense to use oldState, newState because they are a voiceState. As such they do not have a voice property.

So to fix this, all you have to do is use the correct voiceState properties.

const newUserChannel = newState.channelID;
const oldUserChannel = oldState.channelID;

Note: newState.user is also not a thing, however it does provide you with the member object so I suggest you use that instead.

EDIT: Your entire code should look a little something like this.

bot.on('voiceStateUpdate', (oldState, newState) => {
    const newUserChannel = newState.channelID;
    const oldUserChannel = oldState.channelID;
    const textChannel = newState.guild.channels.cache.get('766783720312537089');
    
    if (newUserChannel === '764231813248843806') {
        textChannel.send(`${newState.member.user.username} (${newState.id}) has joined the channel`)
    } else if (oldUserChannel === '764231813248843806' && newUserChannel !== '764231813248843806') {
        textChannel.send(`${newState.member.user.username} (${newState.id}) has left the channel`)
    }

});