Hello Jordache,
 
Thank you for reaching out.
 
I found an article in our knowledgebase that should answer your question, please click on the link below for access to it and to review. See section V to VIII in particular.
 
- KB 10233 Error: “You are attempting to access an area that another user is also using, or you are attempting to perform a process that requires single-user access."

Here's some more resources for Sage 50 US:

Thanks,
Erzsi

Thanks Ms. Erzsi for your support. I have checked the above articles, but couldn't find the errors which i am facing now. i am not able to create or modify the customer or vendors. Can you please advise me, why i am facing this issue or a solution for this issue? 

The error message you are getting can sometimes be resolved simply be restarting all computers that run Sage 50, including the server. If that doesn't work, then there are several possible solutions. Try these knowledgebase articles to see which one works for you:

ID:11778 Unable to save maintenance records

ID:37242 Error "You are attempting to access an area that another user is also using"

ID:10233 Error: “You are attempting to access an area that another user is also using, or you are attempting to perform a process that requires single-user access."