If you have smart posting, turn it on. 

it's ON always. But still, workstations even server gets this error while opening the software 

How many computers did you load the program on and how many user licenses did you purchase? 

Hello Noushad Abdul Azeez, 

Thank you for reaching out. I sent you a friendship request asking for you to share your contact information with me privately, so that I can have someone from our Sage Support team reach out to you directly to provide assistance. 

https://support.na.sage.com/selfservice/viewContent.do?externalId=10233&sliceId=1

10 users license. installed only 9 computer and 1 server

Run all .Net Framework updates from Microsoft Update. I've run into this problem multiple times and when everyone is updated to the latest .Net Framework the issue goes away.

I reinstalled the Sage 50 2020 at the server. now it is working but I don't know the error will come again or not. Anyway, I will update all the .Net too. Thank you

I am working on it. Thanks