Try deleting your web browser's cookies, temp files, history, cache, etc. , then close the browser, reopen it and then try to complete your purchase. No guarantees that will resolve the issue but, it something easy you can do yourself.
Note, it's possible there could be other issues, but as this is public community, no one here can look at user account info. For example, you may have limits on your account for both sending and withdraws (just guessing here) or it's possible the purchase requires a credit card or there's something uncompleted with your account setup.
You may want to try phoning customer service or you an reach out to PayPal via their Facebook or Twitter Account, both are supported by PayPal employees.
