Close but not quite. A Yes/No property can have two values -- True or False -- and those are constants, not string literals, so no quote marks around them. See if you can finish the formula, given that info.

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If i've got a checkbox with yes/no option [Is this a renewal ?]
with a text field beside it [please enter new contract number if it is a
renewal], can I use the following formula to force the user to enter a number
in the text field?

([Is this a renewal ?] "yes") OR ([please enter new contract number]"")