You may only apply one time for the Youth Mobility Scheme. If you have entered the UK on the Youth Mobility Scheme visa previously, you are not able to apply for it again.
For example, if you come to the UK for three months through the Youth Mobility Scheme and then decide to leave and go home for 3 years, you are not permitted to apply and enter the UK through the same category.
If you successfully applied for the visa but never entered the UK (and therefore never activated it), you are allowed to apply for another Youth Mobility Scheme visa. However, you will be required to pay the fee again.