The error message contains the explanation, but maybe this is not so clear (at least, it was not clear for me for the first time):

Multiple items cannot be passed into a parameter of type "Microsoft.Build.Framework.ITaskItem".

So only one PreDeploy or PostDeploy script can be used.

In other words, if you have many scripts in the folder (like on the screen attached) and they are not excluded (removed) from the project in any way, then the builder sees multiple items.