i am not able to download macos big sur as i have man 2015 early 13 inch

Can you please tell us the macOS version installed on your Mac?

Quote:

If upgrading from macOS Sierra or later, macOS Big Sur requires 35.5GB of available storage to upgrade. If upgrading from an earlier release, macOS Big Sur requires up to 44.5GB of available storage.

https://support.apple.com/en-my/macos

Startup in Safe Mode and try.

Startup in Safe mode. https://support.apple.com/en-gb/HT201262