我发现了问题.我已经将 .env 放在 .gitignore 内.因此 .env 在github存储库中不可用，我已将其克隆到我的Digital Ocean液滴中.作为解决方案，我使用 vim 在我的Droplet中重新创建了 .env 文件.这样我就可以启动服务器了.

I found the issue. I had put .env inside .gitignore. Therefore .env was not available in the github repo, which I had cloned to my Digital Ocean droplet. As a solution, I recreated the .env file inside my droplet using vim. Then I could start the server without any issue.