The "Optimise now" button is there if the phone needs optimisation. Swipe up to open the app try, on the search bar type "device" it should show an app called "Device care", tap to open the app.
Below the face, the "Optimise now" button will be there, if it is not there it should say "Great!" meaning that it does not need optimisation.
To clear cache, of an app go to settings:
Settings -> Apps
Scroll to the required app and tap it, the "App info" should open.
Scroll to Storage and tap. It should open the storage page, at bottom on the right there is "Clear cache". That will clear the cache for the app.
To clear the cache partition,
Swipe long from the top, on the top right there is the power button to the left of the settings button.
Turn off the phone.
connect the phone with a USB cable to a computer or another phone.
Press the side/power button and Volume up simultaneously until the samsung logo appears.
The phone should boot into recovery mode. The volume keys are used to navigate and the side/power button to select.
You can see this video to get familiar with the process. By the way, I have no association with video or the maker.
https://www.youtube.com/watch?v=68cEQmetaHk
