You can generate the license by accessing https://my.atlassian.com using your Atlassian account account. To generate a new license perform the following:

1. Navigate to https://my.atlassian.com and log in using your Atlassian account
2. Under the Licenses section, select New Trial License to generate the license
3. Choose the product from the dropdown menu and provide your existing server ID
4. Agree to the terms and conditions and generate the license
5. Copy the new license key to your clipboard

Once the license is generated, update it in your Bitbucket instance by performing the following:

1. Navigate to Bitbucket Data Center Administration > Licensing
2. Select Edit License
3. Paste the new license key and select Save to store the changes