Well I noticed today the same issue in my new sites.
From version 9.8.1 It appears DNN removed ModuleCreator as Core Module, for this reason DNN now have another control for view source of modules, this control allows edition just in local sites (localhost). Fortunately this have solution.
I highly recommend to take a backup for you site before to start
First of all is needed to install the ModuleCreator module if is not already installed. I tried with last versión at https://github.com/DNNCommunity/Dnn...tag/v9.8.1
Once you have installed ModuleCreator module, you need to run a script, to replace current ViewSource control to ViewSource control from ModuleCreator module.
Run this query on SQL Console as host, please be careful to run all script (3 lines), including WHERE part, without this you will damage completely your site. Do i recommend to take a backup after all the process ?
WHERE [ControlTitle]='Develop Module' AND [ControlSrc]='Admin/Modules/viewsource.ascx'
Lastly you need to restart application to load this new information and now you will be able to edit module files again.
LAST NOTE: I´m pretty sure DNN colaborators removed this functionality on purpose, maybe for security, but I think this should be optional, with some additional flag in site or security settings. I will try to request this on github