In this context:
Most of our clients have issues because of their limited knowledge. It is very stressful to repeat the same things to all of them, knowledge that should be basic for them.
I'm told it's considered an ungrammatical phrase, but I don't understand why.
Additional information: my b2 english teacher told me
Here's his explanation:This particular sentence is actually a subordinate clause that provides a sentence element with additional information, but which cannot stand as a sentence on its own. A subordinate clause can either modify an adjacent clause or serve as a component of an independent clause, meaning that it is dependent and requires an additional clause.Moreover, the noun ‘knowledge’ is commonly preceded and not followed by a verb, e.g. ‘acquire knowledge’, ‘have knowledge’, ‘use knowledge’, etc. And finally, the noun ‘knowledge’ does not collocate with the verb ‘to be’ that often, because of the above-mentioned rule. Attached you shall find a screenshot of the Oxford Collocations Dictionary which gives all the verbs the noun ‘knowledge’ collocates with. Now, let’s take a look at the sentence ‘The customers should have some basic knowledge.’ This is an independent clause and thus a sentence that can stand on its own. The noun ‘knowledge’ is preceded by the verb ‘have’ which is at the same time a verb that this noun usually collocates with.