Welcome @Arjun Uttama nambi SN !
Looking at this description what you provided it is very hard to define what kind of error you are having. The link it is not something that everyone here is able to check.
I would start with checking who is the admin of this instance and try to reach him. If you are an admin then would be nice if you can provide more information, some screens etc.
I personally would start with checking you on specific space (Doppler) users that try to upload files (attachments) have permission to do it.
Here is more information about space permissions..
https://confluence.atlassian.com/doc/space-permissions-overview-139521.html
Looking forward to hear from you!
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