Please don’t share pictures of text. Paste and format the text. See point 11 here.
Now I’m going to ignore that advice to show you this:
See how the documentation for the timer option duration says “integer or time”?
That means no templates, and no helpers.
However, what you want to do could be accomplished with an automation as the timer start service does allow templating of the duration. However this will start the timer whenever the duration is changed. If you don’t want that you could pause or cancel the timer right after using the timer.start service.
There’s another problem though. The duration has to be in the format HH:MM:SS and your input_datetime appears to have a date and time. So it will need formatting in the template. You could use an input_number that contains the number of seconds (or minutes or hours) for the timer to run. Much easier.
e.g.
trigger:
- platform: state
entity_id: input_number.duration_in_seconds # this will trigger on any change of the input number
action:
- service: timer.start
entity_id: timer.your_timer_here # I could have cut and paste your timer entity id here but you posed an image and I'm too lazy to write all that with correct spelling. See why pasting images of text is bad?
data:
duration: "{{ states('input_number.duration_in_seconds') }}" # this template could be adjusted for minutes or hours, you could use an input_datetime but that takes more formatting than I'm prepared to do right now as I'm late for bed.
- service: timer.cancel
entity_id: timer.your_timer_here # you only need this if you dont want the timer to start whenever you change the duration.
