I’m not sure whether my bachelor degree is "equivalent to a communication science bachelor"? What does this mean?

It is a legally required step in the admission procedure to check whether the specific degree of an applicant is equivalent to a communication science bachelor as it is issued at the University of Vienna, or whether equivalence can be reached by prescribing a maximum of 30 ECTS in additional courses.
The additional courses prescribed for students with a non-communication science bachelor are usually seminars in social science research methods. These courses are offered within the master program on a regular basis.

I can't provide a bachelor diploma or a final transcript of records because my bachelor is not finished yet. Does this mean that I cannot apply?

The applicant's bachelor does not need to be completed at the time of application. In this case, please include a formal letter issued by your current institution confirming that you will receive your bachelor degree until September 15th at the latest. Also please include a transcript of records of all courses done so far in your current bachelor program.
Successful applicants, who have not yet finished their bachelor degree, will be admitted to the programme on the condition that they provide their degree certificate until September 15th at the latest. 
Please note however that this conditional notification can not be used to apply for a residence permit. For this, a final official notification of admission is needed, which can only be issued as soon as all formal requirements are met.

I've already completed a Master. Can I still apply for this Master program?

Even though you have already completed a Master in different or related field, you can apply for this Master in Communication Science.