This company is looking for a specialist who understands "the" or "-" issues of the environmental...
This company is looking for a specialist who understands "the" or "-" issues of the environmental pollution.
Do I have to put "the" here?

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Personally I would put "the" in. It is more specific and in this context, there may be issues particularly relevant to the company.
Leaving out "the" is also OK but less specific, referring to the general issue of environmental pollution.

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Simon3142 gave an excellent answer.

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you can also book a lesson with me if you want more information about that.

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Hi Olena,
If you are talking about some specific issues, then there should be a "the " before it. 
but if you are talking about all environmental issues, then there is no need for "the".
there are three ways of talking about things generally in English. if you just search "generic noun", then you can find out about the rules.