1. since you've downloaded this third-party GUI from md5decrypter.co.uk it would be much better if you ask GUI related questions there, right? (there is also a forum on that domain - also for this kind of questions)
2. you are using hashcat 0.39 (see here https://hashcat.net/hashcat/ what the recent version of hashcat is)
3. you probably didn't select WPA/WPA2 as Hash Type in the GUI.... just guessing
4. make sure that both GUI and hashcat is up-to-date .... or try to use the hashcat w/o GUI (maybe then it is easier for us to help you..... otherwise ask for support there: md5decrypter.co.uk )
2. you are using hashcat 0.39 (see here https://hashcat.net/hashcat/ what the recent version of hashcat is)
3. you probably didn't select WPA/WPA2 as Hash Type in the GUI.... just guessing
4. make sure that both GUI and hashcat is up-to-date .... or try to use the hashcat w/o GUI (maybe then it is easier for us to help you..... otherwise ask for support there: md5decrypter.co.uk )