I would like to tackle this question from another angle for you.

You have done a literature review for the dissertation proposal. You report your findings from those literature after you study them, so you have descriptive writings in the proposal.

But, what do you propose to research? Have you pointed out any possible issues from the literature? Have you discovered anything? Anything worth researching?

Those are the things they are looking for. They want you to have critical edge in the proposal.

I found a link from University of Leicester about What is critical writing very helpful, at least to me. It contains several sections, What is critical writing? What is descriptive writing? The difference between descriptive writing and critical writing, examples and other useful information.

I would like to tackle this question from another angle for you.

You have done a literature review for the dissertation proposal. You report your findings from those literature after you study them, so you have descriptive writings in the proposal.

But, what do you propose to research? Have you discovered anything? Have you challenged the existing literature? What do you want to research? Why do you want to research that topic? What are your arguments? What would be your own approaches? Why would your approach work? Etc.

Those are the things they are looking for. They want you to have critical edge in the proposal.

I found a link from University of Leicester about What is critical writing very helpful, at least to me. It contains several sections, What is critical writing? What is descriptive writing? The difference between descriptive writing and critical writing, examples and other useful information.