English equivalent of Greek "στο πτυχίο"
Solution 1:
To summarize what has only been noted in comments, educational systems vary by locale, so what is familiar in England may not be applicable in Scotland, what applies in New Zealand may not in Australia, and so on and so forth.
In the U.S., one typically needs to pass 120–130 semester units of coursework over four years to earn a baccalaureate, but you cannot arbitrarily pick those courses. For a typical example, undergraduates at the University of California, Berkeley, must complete four sets of requirements: those set by the University of California system, those set by the Berkeley campus, those set by their college/faculty/school, and those set by the department or its major/concentration program. These requirements are not necessarily academic; Colgate requires two units of physical education, Bryn Mawr requires a swim test, and Cornell requires both.
Therefore, it is possible for someone to have passed enough credit hours to earn a degree, but not have fulfilled all the requirements to graduate from the institution
Most programs are designed so that a traditional student can complete the requirements for a bachelor's degree in four years, graduating after your fourth year. In the U.S., the fourth year of university enrollment is your senior year, and so someone who is still a student after four years may be called a super senior, or fifth-year senior, sixth-year senior, etc. This term is informal, although there are a few examples of official usage, and can carry negative connotations of poor academic performance or a lack of discipline. I would employ it with great caution.
Some may draw a distinction between super seniors (those who have not completed a four-year program in four years) and fifth-year seniors (or simply fifth-year students, especially if the institution does not use the traditional freshman, sophomore, et al terminology), those in the final year of a program designed to be five years long, such as the B.Arch, or programs with a co-op year, or if the career is deliberately extended to play sports for the university through a process called redshirting.
Solution 2:
For what is worth, in Italian there is the expression "studente fuori corso" which is exactly what you describe. I have never come across an equivalent expression in British English: the system does not allow for the situation to arise, hence the lack of an equivalent expression.
I wondered if "off-track" might be used in this context - and a quick search led me to this:
https://catalog.asu.edu/glossary
It doesn't seem to completely match, but it seems close enough.