Note the difference between this and אני מדברת עברית ‘I am a Hebrew speaker’. This noun/adjective is also used in, say, סֶרֶט דוֹבֵר עִבְרִית (‘a Hebrew-speaking film’).
Because Duo. makes an answer incorrect if one use the word 'a' incorrectly in place of 'an' or visa versa, I wanted to point how much this needs to be corrected. Here in this sentence if you use 'an' which is grammatically correct you get marked wrong. If Duo. just marked such minor mistakes in English, correct and mentioned that there was a spelling mistake or a grammar mistake, that would solve this problem. I was told last time I wrote about this that the correction is automatic an there is nothing to be done about it ... Maybe there is something that could be done...
"An" is not gramatically correct in this exercise. Whether to use "a" or "an" in English is determined not by the first letter of the next word but by the sound. If it is a vowel sound we use "an", including when the first letter is a silent "h", like in the word "herb". The "h" in "Hebrew" is not silent, so we correctly use "a" and not "an".