Activities are complex to analyze, execute, debug and maintain. As a best practice, alternatives such as Flows, Data transforms, Declarative rules are considered. The same warning is displayed while creating the activity. You can click Here for more details on the same.
1. what is alternative cases and where we can use that alternative cases and uses?
The Alternate Stage is something that is deviating from the normal Case Stage. Like a user decides to resolve the case as a duplicate of another case (or) Case gets withdrawn when a particular SLA is met.
2. can we have call datapage in datatransform?
Since Data Pages are loaded declaratively, you just have to refer them in any rule. If you would like to load it procedurally, then you have to use Load-DataPage in Activity.
3. what error message we get when we create an activity?
I am unable to decipher this, Can you please rephrase it a little ?