After Effects is very important to me , I haven't been able to work using it as it gives me the following error:
Please I need help on this!!!
Thank you on advance
You need to update both AE and Yosemite. There were some big bugs that were fixed in 10.10.5 that affected After Effects. And your AE should be on version 12.2 if you want the least buggy version of CC.
Although, that's a somewhat old version of After Effects.
After you update your OS, you might also consider installing CC 2014, CC 2015, or CC 2015.3 versions of AE.