TL;DR: If you do not find anything to criticise on a paper, strongly consider the possibility that you were too coy.
If I give a paper of mine to a bachelor student for internal review and they do not have anything to criticise, I assume that they did not thoroughly read it, underestimated their concerns, or are afraid of criticising senior collaborators.
This particularly holds if they are a co-author.
(And this also applies to peer reviewers by the way.)
Every paper that has not already received feedback by thousands of people will be imperfect.
It does not necessarily have to contain hard mistakes, but there almost certainly are parts that can be misunderstood and explained better – not to mention spelling or grammar flukes.
The challenging part of writing a paper is not to correctly write down your results; almost everybody¹ can do that.
The challenging part is to explain your rationales, methods, and conclusions to readers that may not be extremely familiar with all the details of your research.
Therefore taking this reader’s point of view is essential for good paper writing.
However this gets increasingly difficult if you are very familiar with a subject, such as your own research.
Therefore somebody like you is probably a better proxy for the paper’s intended audience and a better person to provide feedback on the paper.
This does not only apply to subject expertise but also to language:
I often write sentences that I consider to be to the point and easy to understand, just to find out that my internal reviewers had to read them three times to understand them – which means that another reader may not understand them at all and they need to be fixed.
If your seniors are wise, they are aware of this and will take any criticism coming from you seriously, not only regarding the paper’s didactics but also regarding possible mistakes:
If anything seems fishy to you, it may also seem fishy to reviewers and the paper may need to be improved, be it by fixing an actual mistake or by better explaining why there is actually no mistake.
Also, consider that your seniors may judge you like I did if you do not provide any criticism whatsoever.
You may even waste their time as they may feel the need to consider another internal reviewer due to your lack of criticism.
Therefore I strongly suggest that you carefully read the entire paper again and be as nitpicky as you can, in particular with respect to easy understandability, consistent notation, spelling, and grammar. You will almost certainly find something.
¹ who is capable of obtaining publishable results