I need someone to confirm if this sentence is correct.

The sentence is; Lertaldë ala ista si lambelë

I'll comment what I intended it to mean once someone responds so my intention doesn't change how you guys translate. Thank You!!

My guess is you wanted to say "You cannot/may not know this language."

If so, there is a small issue with the negation there.

Your right! In hindsight I think I was going for something like, 'You cannot decipher this sentence'. Is that, Lertaldë ala si hanya-quentelë (I think that means 'You cannot comprehend this sentence'?

“Can” or “cannot” is translated with multiple different words in Quenya:

• polin quete “I can speak, I am able [physically] to speak”
• istan quete “I can speak, I know how to speak”
• lertan quete “I can speak, I am free to speak”
• ece nin quete “I can speak, There is a chance for me to speak”

So you need to decide why we can’t understand your sentence, I assume “You can’t because you lack the relevant knowledge about Quenya”, so ista- (not lerta- “I can’t understand because I’m forbidden to”). With that I’d say ú istalde hanya sina quettale/quetie. (← ú “not”, ista- + -lde “y’all know”, sina “this”, quetale “sentence, expression”, quetie “words, *saying”)