I'm afraid there's not a lot we can tell you here.
You have tagged this with Bitbucket Cloud, which implies that you have set up something on Bitbucket that is trying to connect to a database, but you have not told us what you have configured, so we don't know what you are trying to do (it's not the core of Bitbucket Cloud throwing this error, Cloud doesn't use MySQL)
If you've mis-categorised this, and you're actually on Bitbucket Server or Data Center, then the "what have you configured" question is still there, but it might also apply to the core installation - Bitbucket needs a database to store its data, and your error message could be coming from the installation itself.
So, can you tell us where you are actually getting this error message? What are you doing that provokes it?
We can also start on an answer - the message really does mean that the service can't connect to the database - it's not running, it can't be reached over the network, or the credentials you've told it to use are wrong (unknown user, wrong password, don't have permission to use the database, etc)
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