In general, yes, asexuals are physically capable of having sex. Asexuality is a sexual orientation and has no bearing on sexual ability. There are no physical characteristics inherent in asexuality. Asexual people typically have functioning genitalia which is indistinguishable from that of a non-asexual person. It is possible that an asexual person is physically unable to have sex, but if that is the case, then it is the result of some other condition and not the result of asexuality.
Now, that’s not to say that asexual people necessarily want to have sex. Physical ability and willingness are two very different concepts. Many asexuals, despite having functioning genitalia, have no interest in using that genitalia with anyone else.