It seems OptiFabric dousn't see the 1.16.4-OptiFine_HD_U_G5.jar in the mods folder.

Opening wich Jar file? The OptiFabric or the Optifine one. Also where do I find a log file to provide to you?

try opening the jar file, extracting it, and trying again, also please give a log

With you guys's help I found the root of the issue... When you re-use the Jar file that you've used to launch Optifine as a version instead of a mod it expands itself to 17 MB and it becomes unuseable to OptiFabric. Now that I've used a fresh version it works perfectly. Thanks for the assist

optifine, you can find log in .minecraft\logs\latest.log

That screenshot looks suspicious — OptiFine jar downloaded from the official website is named OptiFine_1.16.4_HD_U_G5.jar, not 1.16.4-OptiFine_HD_U_G5.jar.

oh yeah did not notice that; where did you get optifine?

Also, official OptiFine jar is 5 MB in size; it is 17 MB on the screenshot. I bet it is a malware. Upload it to VirusTotal to verify.

That is quite interesting, I did get it off of the original website... Here is a screenshot from the results of the website you posted:

https://optifine.net/adloadx?f=OptiFine_1.16.4_HD_U_G5.jar&x=fb0d is where I got it from.
Is a preview of the pop-up my browser (Firefox) gives me when I want to download it. Not the syntax you seem to be used to with the file name.

interesting, how did it get to the file name you're using? anyway download it from that site again and put that one in