Any message error containing the phrase "outdated server" normally means one thing in particular – A player is trying to join a server with the wrong Minecraft game version. ... It makes joining for the player impossible as the server does not support the game client version that is trying to connect to it.

Why does my Minecraft say could not connect outdated server?

When the server is not the same version as your Minecraft client you'll see an “outdated server” message when you try to connect. This means you need to switch to the same version that the server is on, in order to join us. The good news is, with the current launcher, it's extremely easy to switch between versions.

Can't connect to realm outdated server?

The reason why they can't connect to your realm is that they are either using an older or newer version of the game. In either case, they will have to download the same version as the one installed on your PC. Doing so will result in you and your friends being able to play the game in the same realm.