I want to write "If left unticked ......" , but it seems like unticked is not a word(or is it?), what would the correct opposite of ticked because I use "If ticked ...." at the beginning of paragraph.
The word “unticked” is found defined in the Cambridge Dictionary but not in the Merriam-Webster online dictionary. It is not used in American English and the equivalent word is “unchecked”.
Normally you would say either,
I left the tick box unticked. BRITISH
I left the checkbox unchecked. AMERICAN
It may be regarded as strange by many native English speakers to mix the different usages.
However, in Australian English “tick” is preferred but “check” is very possible too particularly in “checkbox” as it’s what these boxes are labelled on websites and in popular spreadsheet applications. So, “Leave the checkbox unticked” sounds like perfectly acceptable English to me.