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To get a work permit as foreigner /citizen from a country outside EU/ is relatively hard thing to achieve. The employer you are working for needs to have permission from the General labor inspection to hire you. And such permission will be granted only if your qualifications are such that no one Bulgarian citizen can not do the work you are hired. In my practice I have difficulties getting a permission even for Chinese citizen who were a translator for one of the rare Chinese dialects.
If you own a company, or you are a manager of a company it won't do you any good either, because you'll need to hire 10 bulgarians to work in the company on fulltime basis.
The only thing that can help you in receiving prolonged stay is to be a manager of trade representative organization /TRO/. If you want to do that you'll need to have a company you own in your country, to establish its TRO in Bulgaria, to apply for Visa D in the Bulgarian embassy of your country of residence and after that a prolonged stay will be granted by Bulgarian migration authority.

Are you permitted to conduct freelance work activity by being a manager of trade representative organisation?

You are just representative you can't work on labor contract if you don't have permission from the Labor department. But you can provide services through other company you own and manage. TRO manager is just for residence status.