Sometimes, especially after switching themes, WordPress will generate 404 errors on the category and tag pages. There might be multiple reasons – and also multiple solutions – to this, here is simple solution and it will fix the issue
Go to your WordPress backend, click Settings, click Permalinks. Make sure the category base is “category” and the tag base is “tag” (unless you have some VERY weird special settings) and simply press the “save changes” button. Even if nothing has changed, clicking the button will probably make your category and tag pages work again now.
Oh, Finally Don’t forget to clear the cache…. 🙂