hi Ainul,
you cant answer it in one thread. the problems can be very from the status of your project. if you are in implementation project than it could be problems related to process map, gap, configuration etc.
but if you are in support project,than I can say that there are three common tickets.
1. missing master data or wrong master data - most of the support error occur due to this.
2. if it comes to transactions error than it could be a qty issue or date issue.
3. for support project you may have the configuration problem. but these scenario occurs very rarely because you have configured everything and working fine. so in that case the it will be last things you be looking at.
better you read different threads on sdn and find out the problems, what other consultants are facing.
regards,
Solomon