Action 'Set the Status of a Case' raises error with triggers 'Daily trigger for case activity' and 'Activity added'
The initial problem as stated above is that the combination of Linked Trigger = 'Daily trigger for case activity', Linked Condition 'Days since Last Case Activity- = 90 days, and Linked Action = 'Set the Status of a Case' raises the error 'This action is not available with trigger Daily trigger for case activity'
I tried a variation on the suggested workaround by Rebecca - I first made a rule that adds an activity to the case, like 'made inactive by system', then a second rule as suggested by Rebecca that sets the case status. I tried this but at the end met a similar error message to before 'This action is not available with trigger Activity is added'.
So it seems like action 'Set the Status of a Case' is incompatible with (at least) both triggers 'Daily trigger for case activity' and 'Activity added'