I had a look on the Internet and came across both "build a rapport" (eg. I enjoy building a rapport with customers) and "build rapport" (eg. I enjoy building rapport with customers). It seems to me "rapport" is used by native speakers sometimes as a countable noun and sometimes uncountable. What's the difference in meaning between them?
The article is optional in to build [a] rapport, but most writers don't include it...
In rapid speech it wouldn't always be easy for the audience to hear if it was present or not, but it makes no difference to the meaning in any case.