Blank page appears when i click customise

Hey,

I have been editing my site for many hours now and i worry that i may have deleted something i shouldn’t have. When i click on “Customise” and many other links on my dashboard it brings me to a screen saying it is “Unable to handle this request”.

The page i am trying to view is https://intransitcreations.com/wordpress/wp-admin/themes.php

Any help would be appreciated, many thanks.