Can I do a master's in a different field from my bachelor's?

Sometimes, but it depends on the programme's admission regulations, not on a general rule.

• Most master's programmes in Germany are consecutive (konsekutiv): they build on a bachelor's in the same or a closely related field. Their admission regulations (Zulassungsordnung) usually require a minimum number of ECTS credits in specific subjects, for example 60 ECTS in economics or 30 ECTS in mathematics. If your bachelor's does not contain them, the application is rejected or you are asked to complete bridging modules first.
• Some programmes are explicitly open to graduates of other fields (interdisciplinary programmes, conversion programmes, many management, data science or sustainability master's). These say so in the admission regulations.
• Continuing-education master's (weiterbildend) often require work experience instead of a specific bachelor's subject and usually charge tuition.

How to check: open the programme's Zulassungsordnung or "admission requirements" page and compare the required ECTS by subject with your transcript. If it is borderline, e-mail the programme coordinator with your transcript before applying. Universities decide case by case; there is no national equivalence rule for switching fields.