This is a tricky question.
Yes, it is possible to insert a comma there, but the phrase will have a (slightly) different meaning then:
"hezký malý strom" is about a small tree which is pretty (as opposed to small trees which are not, like "ošklivý malý strom")
"hezký, malý strom" is a tree which is "pretty" AND "small" (at the same time; both adjective are of the same level of importance)
According to this link, it seems that it works quite similar in English: http://www.getitwriteonline.com/archive/042301comcoordadj.htm
FWIW, "small pretty tree" would sound a bit strange, and a comma could just as easily be used there.
In an earlier version of this exercise, absence of the comma resulted in many rejected translations due to the ambiguity factor.
Also, using a comma between adjectives is not unusual. For example, "It was a coordinated, multi-agency effort, but it did not achieve its goal" -- "That man wants to build a tall, impenetrable wall to keep people from entering the country."
We try to explain such things, when questions arise, because many of our learners are not native speakers of English, so they often learn something about English that they did not know before.
Hezký is from Proto-Slavic gъd-jь-kъ, it is related to the verb hodit (to fit), from Proto-Slavic godìti (to please). It is actually related to English 'good', so if you need a frame of reference to remember this word (as I do), what is pretty is what is pleasing and fitting, thus what is 'good.'
In/animateness matters only for the masculine gender. Basically, persons (člověk, muž, přítel...) and animals (pes, kůň, had...) are animate, whereas inanimate applies to the rest - objects (hrad, stroj...), plants (strom...), abstract nouns (úspěch...) etc.
There are some disputable or strange cases, but I think you don't have to worry about them right now. Purely for fun, I'll mention some funny examples of animate masculine nouns: nebožtík (a deceased person), sněhulák (a snowman), strašák (a scarecrow). So apparently, considering the last two words, terms for humanlike (or antropomorphic) objects can be animate as well.
hezký - Nice - Bonito/Lindo/Agradable(A la vista)
krásný - Beautiful - Hermoso/Precioso
Ejemplo: Ves un carro que no se te hace feo, pero no es lo que mas te gusta, "Ese carro esta bonito(Hezký)"
Ves el carro de tus sueños "Ese carro esta hermoso(Krasný)"
Esa es la distinción entre Hezký y Krasný como la entiendo yo.
"Pretty small tree" tests "green," which means it should have been accepted (even without the comma shown in the translation above.) If that was your exact answer, you may have run into the intermittent Duo grading bug. It is helpful to use the Report button to send your answer into the system, if you are fairly certain that it is correct, because we can then tell you what may have been wrong.
I answered like 'pretty small tree' too. I don't know, maybe I started to confuse 'good' and 'pretty' as a translation for 'hezký', but I marked my answer as right using the Report button. Considering the comment of mod, I'm pretty sure that I was right.
Hmm. There is no current report in the system for "pretty small tree." In this sentence, though, the comma helps avoid confusion on the English side between "pretty" as in "nice-looking" and "pretty" as in "very." But Duo generally does not pay much attention to punctuation, and, in any case, I just I just tested it again, and it still comes up "green," i.e., acceptable. Strange.
OK, I did - nice, small tree - as in That is a nice, small tree for the space I have. It was not accepted and I was told it had to be pretty. I know we've used nice for hezký in other exercises. This was just a practice, but I did report it and I would like to know why it was wrong.