why i can't add issue into my sprint??

[Shovon]

After creating sprint when i tried to add issue then there is a error "Something's gone wrong. Refresh the page and try again."

Its a purchased software with 4-5 team members. If its going to happened continuously i am afraid i can't continue jira. 

[Kishan Sharma]

Welcome to the Atlassian Community @Shovon Since you are already using standard plan, I would suggest you to raise a support ticket with Atlassian so that they can assist you with this.  They can check the logs to see what could be the issue and suggest accordingly.