The very same metaphor exists in French since at least the 14^th century and is still in common usage:

Il faut battre le fer tant qu’il est chaud.

The preposition might be tandis, quand, pendant, or tant.

Pendant was most common in the 20^th century but tant seems to take a small lead in recent years.

Your attempt Faites-le tant que vous le pouvez doesn’t convey the same meaning. It’s more like "Do it as long as you are able to do it."

You might say instead:

Faire quelque chose lorsque les conditions s’y prêtent.

Profiter d’une / saisir une occasion² quand elle se présente.

¹ _{Instead of occasion, the word opportunité is often used to translate "opportunity" but is sometimes considered an anglicism even while this meaning already existed in French (See the OQLF).}

The dictionary Le Petit Robert 2006 gives a French idiom (under the entry "battre") that corresponds exactly with the English "strike the iron while it’s hot":

Battre le fer pendant qu’il est chaud: profiter sans tarder d’une situation propice.

In other words: taking advantage of a favourable situation without delay.

Also, "l’occasion fait le larron" is what is as close as "Do something while the conditions are right" I can think of. The meaning is to seize opportunity while it stands