• The subject of this sentence is the entire noun clause: "That there exists no recourse for citizens if environmental violations are condoned at the ministerial level".
• When a whole clause acts as the subject of a sentence, it is treated as a singular subject. Therefore, it requires a singular verb to maintain subject-verb agreement.
• Incorrect: That there exists no recourse... underline a systemic erosion...
• Correct: That there exists no recourse... underlines a systemic erosion...
Corrected Sentence
"That there exists no recourse for citizens (1) / if environmental violations are condoned at the ministerial level (2) / underlines a systemic erosion of (3) / constitutional accountability. (4)"