I play mostly on version 1.8, but I can not start Replay Mod recording. I have everything set up correctly. I tried to upgrade to version 1.8.9, but still the same problem.
Please post the contents of the fml-client-latest.log file inside the logs folder inside your .minecraft folder to some pastebin site and link to the post here.
Note that this log will only contain information on the last time you started Minecraft, not any previous runs.
After hitting the save button, the link should change from https://paste.replaymod.com/ to the same thing but with some additional characters at the end. Make sure you open that new link, not the original one.