Thank you Findingnemo.
Question: My friend has an account in something called advcash and I know his account number (wallet). But when I tried to register on that site, I couldn't because it said that Advcash is not allowed for U.S. citizens. But would I be able to send Bitcoin from my Coinbase account to his advcash account? Or would I be prohibited from doing that because I'm a U.S. citizen?
The following is presented as general education on the use of Bitcoin and steps to consider to avoid loss of funds. Note, however, that if you are using Bitcoin to attempt to circumvent some restriction that is specifically intended to prevent the transfer of funds between the two of you, then doing so could potentially be considered "money laundering" in whatever jurisdiction you are currently in. Since you have now posted in a public forum that you are attempting to accomplish this task, you have increased the probability that law enforcement officers may search you out and that you could be prosecuted. As such, it is only advisable that you continue in this effort if you are either certain that you are not attempting to circumvent a law or that you are comfortable with the risk you are taking.
That being said:
If AdvCash is a legitimate service, and not a scam, (I don't know much about them) and if AdvCash provides a Bitcoin receiving address for your friend, then yes, you should be able to send Bitcoins from Coinbase to AdvCash. Coinbase doesn't generally know anything about who or what service owns any particular bitcoin address. As far as they are concerned, a bitcoin address is a bitcoin address. As long as it is a valid bitcoin address, Coinbase should be able to send the transaction.
Note that if the bitcoin address is known to be associated with some criminal activity, then it is possible that Coinbase might refuse to send the bitcoins to the address. Furthermore, if Coinbase does send the bitcoins and it turns out that AdvCash is a scam, then there is nothing that Coinbase will be able to do to recover the funds once they are sent.
Also, it is very important that the address you send to is the exact correct address. If your friend has any malware on his computer or phone, or if you have any malware on your computer or phone, then it is possible for that malware during any copy&paste operation to substitute in a replacement bitcoin address that is not the one for your friend's AdvCash account. Double check with your friend and make sure that the address that you enter into Coinbase to send to is for certain the address that your friend sees in his AdvCash account.