Will my paper be rejected if I cannot suggest peer reviewers?
The structure of the cover letter of my journal demands a list of the colleagues who have checked my article. But I don't have any such colleagues.
You might have misinterpreted the journal's instructions. I suggest you go through the journal guidelines carefully and see what exactly the journal wants. I don't think they would want the names of colleagues who have already checked your paper. Probably what the journal wants is a list of potential reviewers from your field field. It is not uncommon for journals to ask authors to suggest reviewers. Some journals make it mandatory while for others, it is optional. If this is mandatory for your target journal, you will have to provide the list; else your paper might be rejected or returned on the grounds that it does not meet the submission requirements.
When a journal wants you to suggest reviewers, they want a list of relevant people who would be able to provide objective and constructive criticism of your paper. A good place to look for suggested reviewers would be your reference list. It is a good strategy to suggest names of authors whom you have cited, as they would have substantial knowledge of your field. You can also ask your colleagues or your supervisor if they can recommend any names.