It looks like other journals have had this problem as well - in an earlier thread about the issue, the Text Editor Extras plugin seems to help. Please give it a try.
I am having the same problem in OJS 3.4.0.4, but enabling Text Editor Extras does not help. I still do not see the code button. I tried to enable all the options in Text Editor Extras, and still do not see the button in Author Guidance.
I would be grateful if someone could help (@asmecher ?)
Thank you
The Text Editor Extras plugin will only affect some TinyMCE controls (using the older JS stack, as opposed to the newer VueJS stack). For the Author Guidance area, look at SubmissionGuidanceSettings.php.
Regards,
Alec Smecher
Public Knowledge Project Team