Sounds like you've uploaded a new payment module incorrectly.
Modules consist of two parts: the logic module and the language defines, both of the same filename, but loaded into different folders. If you skip one of the files or if you swap them around, you'll cause a PHP error, which will stop the page from loading, which thus results in there being no "edit" button because the sidebar doesn't show.
Possibilities:
- missing or reversed files
- partially-deleted module without properly deleting both required files
- renamed an old version when uploading another one, thus resulting in a mismatch because you didn't also rename the language file ... better to delete or to rename with something other than ending in ".php"
Details can be found in your myDebug-adm-xxxx.log files: https://www.zen-cart.com/tutorials/index.php?article=82