score:2
You should be fine.
Travel from Canada to Germany is at the moment relatively easy.
Canadian residents have unrestricted access to Germany (https://www.bmi.bund.de/SharedDocs/faqs/EN/topics/civil-protection/coronavirus/travel-restrictions-border-control/travel-restriction-border-control-list.html#f13919566)
It's unclear why you did apply for a Visa. Canadians don't need on for Germany unless you want to stay for more than 90 days.
Canada is also NOT on the list of risk, high-risk or virus variant countries https://www.rki.de/DE/Content/InfAZ/N/Neuartiges_Coronavirus/Transport/Archiv_Risikogebiete/Risikogebiete_2021-08-20_en.pdf?__blob=publicationFile
Since you are fully vaccinated and are not coming from a high risk area you don't even need a Covid test (https://www.lufthansa.com/us/en/corona-tests.html)
What would be the possible causes that might prevent me from entering the country?
Overall Germany is relatively easy going entering Germany is much less of a rigmarole than entering (for example) the US.
for some reason I might not enter the country and have to flight home
That is very unlikely. If things go sideways, it typically happens at check-in. If your paperwork is not sufficient, the most likely scenario is that you will be denied boarding and can't get on the flight in the first place.