It’s an early 2015 Mac book pro and the website says it’s compatible
We understand you're unable to download a software update.We're happy to help.
To get started, we'd suggest providing more specific detail about the issue you're experiencing. What is happening when you try to download this update? Does the download not start when you click on the update prompt in System Preferences? Does it start downloading, but doesn't make any progress? Are you getting some sort of error message? etc.
Answers to questions like these will help give the community a better understanding of what's going on so members can provide more relevant responses.
In the meantime here's some basic info on updating that may help: How to update the software on your Mac
We look forward to your response. Cheers!