M. Quintilian speaking of rhetoric and stylistic blemishes in the works of great orators:
Modesto tamen et circumspecto iudicio de tantis viris pronuntiandum est, ne, quod plerisque accidit, damnent quae non intellegunt. Ac si necesse est in alteram errare partem, omnia eorum legentibus placere quam multa displicere maluerim. (Q. Inst. XX, 24.)
Loeb translation by H. E. Butler:
But modesty and circumspection are required in pronouncing judgment on such great men, since there is always the risk of falling into the common fault of condemning what one does not understand. And, if it is necessary to err on one side or the other, I should prefer that the reader should approve of everything than that he should disapprove of much.
The expression itself should not, however, be ascribed to Quintilian; it was rather a common proverb at the time.
It seems to me, therefore, that your translation damnant quod non intellegunt is not only grammatically correct (which can be ascertained without even a recourse to Quintilian) but also perfectly literary.
Thank you. I read that only capital letters were used, and instead of spaces, they used periods back then. Someone also mentioned that the letter "U" doesn't exist in the Latin language, and would translate as:
Is this correct? Would there also be a period at the end?
Damnant is correct. Damnent is coni. praes. and it would mean "let they condemn". In the quote above, there has to be coni. praes. because of the coniunction "ne", but on your tattoo, you don´t want it, you need an indicative.
ioanna_ioannina already noted that your original use of damnant is correct. quae is plural of quod; in this context you can use either you like.
Romans did not have the letter U, that's correct.
As for spaces or dots to separate words, there were multiple and rather inconsistent systems at different times and for different purposes of writings. Using spaces, dots, periods to separate words, as well as stringingthemalltogether would not make it incorrect or illegible.