said 90% of motorbike mechanics in Vietnam..
shouldn't it be 'professionals'? since it is 'we'
I think in the above sentence "chuyên nghiệp" is used as an adjective not a noun
yes, and the noun professional could be translated as "người chuyên nghiệp"
but if it's being used as an adjective, it needs to be modifying a noun. It's not, however, in this sentence. i.e. We are not professional actors; We are not professional athletes.
"professional" is predicative here and is modifying "we".
Why don't we put không phải là?
Là is used to connect noun phrases. This is a noun and an adjective/verb.