mimemagic has been yanked
mimemagic(0.4.0, 0.3.6 and previous versions) has been yanked, and it is not available for download directly or for other gems that may have depended on it.
Your bundle is locked to mimemagic (0.3.6), but that version could not be found
You need to update
caxlsx gems which dependent on
mimemagic to the latest version.
Gemfile & Gemfile.lock
gem dependency mimemagic --reverse-dependencies
Then, edit and modify
# Rails 5.2.5, 126.96.36.199 and 188.8.131.52 have been released | Riding Rails
bundle update command to update `Gemfile.lock’.
 GitHub - caxlsx/caxlsx: xlsx generation with charts, images, automated column width, customizable styles and full schema validation. Axlsx excels at helping you generate beautiful Office Open XML Spreadsheet documents without having to understand the entire ECMA specification. Check out the README for some examples of how easy it is. Best of all, you can validate your xlsx file before serialization so you know for sure that anything generated is going to load on your client’s machine. - https://github.com/caxlsx/caxlsx