Will a letter of recommendation from Amazon, where I completed a course but did not work, be accepted in Germany for a Master's?

No, a letter of recommendation from Amazon based only on completing a course - without any work or internship experience - will not be considered strong for your master's application in Germany.

Admissions committees expect LORs from people who have directly supervised your work or academic performance, such as college professors or internship supervisors. A course completion certificate or letter from Amazon can be included as a supporting document, but it cannot replace a detailed, personalized LOR.

For the best chance of admission, focus on getting recommendations from those who know your abilities and achievements well.