Can I take a cutting from a plant in flower?

Yes, but a cutting taken while the stock plant is flowering will need to be forced to revert back to a vegetative state under 24 hour light. This causes additional stress, slowing growth and development. Rooting may take up to three times longer when the cuttings are taken during the flowering cycle. The cutting may not have enough stored nutrients or carbohydrates to survive. If you must do this then take the cutting when the mother plant is only a week or two into flowering.

Additional comment...

The above "flowering plant" is to be taken verbatim.
However, the above should not to be confused with a sexy mom who has been showing her primordia aka pre flowers while vegging, and has been kept in a vegetative state of growth for several months or years. These old mature primordia displaying moms clone just as well as a premature seedling.