Richard Branson might have said that if somebody offers you an amazing opportunity but you are not sure you can do it, say yes then learn how to do it later, however for some of us the idea of not being fully prepared can induce more fear than a horror film and rather than propelling us forward can leave us standing still with our mouths open.
Whether you're an expert in your field or just starting out, there is always the possibility that you'll be asked to do something completely out of your comfort zone and with new technologies being introduced at a faster and faster rate, that possibility just got a lot higher. Learning how to deal with tasks you're not familiar with has become a vital skill, with a strong ability to adapt no longer just a phrase to bulk out your CV.
A mistake often made is to pass the buck to somebody else, leaving them to work out the task, however in doing this, you're missing out on a crucial opportunity to demonstrate your value and learn something new. Requesting time to look into a problem and using that time for research is a much better reaction and is more likely to impress your colleagues.
A vital skill in the workplace is to recognize where your weaknesses lie before the opportunity arises that you must use them. Founding Partner of Proteus, Erika Andersen, calls this "managing your own growth", something she says will make you much more appealing to your employers. By requesting training, you aren't showing your weaknesses but rather demonstrating your strength of character and urge to grow as a more valuable worker. If you don't feel comfortable telling your employer that something has you stumped, there are other avenues to go down. If good old Google doesn't do the trick, could you find a mentor, enroll on an online course, or find somewhere where development is encouraged, not discouraged?
Admitting you aren't 100% sure isn't a flaw, not everybody can know their specialism inside out, however the most important factor in responding to an unknown request is to offer a solution, even if that solution is simply to look into it. Don't act despondent, just act.