Find the part of the sentence that contains an error:

Although numerous corporate entities modified outdated guidelines (1)/ to restrict workplace complaints not only demonstrates executive overreach (2)/ but also undermines the collaborative environment (3)/ that these policies were meant to foster. (4)

The correct option is (2).

Step-by-Step Analysis:
To find the error, let us analyze the sentence structure section by section:

1. Part (1): "Although numerous corporate entities modified outdated guidelines"
This part begins with the subordinating conjunction "Although", which introduces a dependent (subordinate) clause. Any sentence starting with an adverbial clause introduced by "Although" requires a complete main (independent) clause to follow it, containing its own explicit subject and verb.

2. Part (2): "to restrict workplace complaints not only demonstrates executive overreach"
This segment contains the grammatical error. After the infinitive phrase "to restrict workplace complaints", the sentence immediately uses the predicate "not only demonstrates executive overreach" without introducing a subject for the verb "demonstrates". A subordinate clause starting with "Although" cannot act as the subject of a main verb.

3. Part (3) & Part (4): "but also undermines the collaborative environment that these policies were meant to foster."
These parts correctly use parallel structure with the correlative conjunction pair "not only ... but also ..." connecting the verbs "demonstrates" and "undermines". However, because Part (2) lacked a main subject, the sentence structure as a whole is incomplete.

How to Correct the Sentence:
A main subject must be added after the dependent clause to make the sentence grammatically complete. For example:
"Although numerous corporate entities modified outdated guidelines to restrict workplace complaints, this decision not only demonstrates executive overreach but also undermines the collaborative environment that these policies were meant to foster."

Thus, the part containing the error is (2).