Slitherine

Is there a way to play/practice the multiplayer sceanrios against the AI?

"More or less" but that means first building up, or selecting, this particular AI... then you could use/play this scenario using its own map and objectives, but then you would lose some part of the replayability, since there will be less adaptations in front!

Specifically, you would have to go through the Editor: load the scenario you want to modify from there.
Then, each unit (that the AI is about to control) has to be created (spawned, pre-deployed, or something). Each unit must then be assigned to an "AI Team" (with relevant orders).
As far as I know, the AI can NOT purchase units; its RPs will only be used to replenish damaged units... so everything must be predefined, in a way.

If the goal is above all to somehow train for multiplayer, then learning how to play well the campaigns already teaches a very solid foundation... there are the official DLCs, of course, but also all the content created by the community - and there is some truly excellent content, completely free! (Just in case, that's where you need to dig a bit, among the topics: viewforum.php?f=374 )