"Do you have time this afternoon?"
The problem is with *这个下午. While it is understandable what you mean, people just don't say it that way (it is permissible in some non-Mandarin local languages, but not in Standard Chinese or an other variant of Mandarin I have encountered so far). You say 今天下午 "today afternoon". And similarly for other days or times of day: 昨天晚上, 明天中午, 那天早上 etc.