If they finish within the top 2, then they can qualify for the main tournament. Only other team (apart from Turkey) who can spoil Croatia's chances is Wales. They are one of the most under-rated teams within UEFA and on their day they can make life difficult even for stronger sides such as England and France. But I don't know how they are going to perform this time, especially after the retirement of Gareth Bale. The other teams in that group (Armenia and Latvia) are there just to make up the numbers. During recent times, their quality has gone down and I don't think that they stand any chance against teams like Croatia, Turkey and Wales.
I know than first two spots in the group lead to the final tournament. But my opinion is that it's not the same to finish first or second. Finishing at the first place boosts team's morale and their synergy with the fans. I remember the time when Croatian fans were not so interested in the national team. Then Dalic was appointed as the manager and they started winning strong teams again. Now the stadiums are again full when the national team is playing.
At least on paper, Croatia is much stronger than Turkey and Wales. The other two (Latvia and Armenia) are there just to make up the numbers. And then chances of any upsets are low, because each team will face others two times in the group phase. Even if one of the underdogs win one match by luck, the higher ranked team has another chance to get back at them. In two weeks, Croatia will be playing their first match, against Wales (they will be having the home advantage, since the match is scheduled to be held at the Stadion Poljud in Split). And on 28th they have a tough match, against Turkey.