How can I get a bachelor's degree if I have an associate's?

You can go to any community college, enroll as a "non-matriculating" (non-degree-seeking) student, and take the classes you need. 

You can actually enroll as a non-matriculating student in nearly any four-year college or university, but often there's some limit on the number of courses or credits you can take. A community college won't limit them. 

Complete the prerequisites.  Go to a college and take the classes individually.

Can you transfer into a 4-year university as undeclared major, complete all the digital marketing prerequisites, then declare your major? Also, digital marketing is in vogue, but rather limited. Depending on your talents, abilities, skills, etc, you might consider a more broadly based major, then specialize.

Take the prerequisites.  It's that simple.
Find a local college that offers them and take them.

You could still sing up for classes that aren't needed and take those classes. Or you can start over and do it. 

If you think your existing qualifications and experience are roughly equivalent to the prerequisite classes, you could contact the leaders of the Bachelors course you want to join to ask if they would admit you.  They are unlikely to accept you on the course if you lack too many of the required qualifications as your chance of success in the course would be reduced and failures make them look bad.

Transfer and take the prerequisite classes at the university. Then go into your major.