If you're seeing this confusing WordPress error when trying to upload the theme, it means the process breaks at some point and that's most probably due to a few server settings limiting resources for your site. For example the maximum upload size could be set to 5MB and the theme's package is larger than that. This isn't a bug but a matter of server settings.
There are two ways to solve the issue:
- Reach out to your hosting company and ask them to increase the upload size limit and the PHP memory allocated for your site.
- Connect to the server and manually do it yourself: How to increase memory allocated to PHP
Recommended PHP configuration:
- max_execution_time: 60
- memory_limit: 128M
- post_max_size: 32M
- upload_max_filesize: 32M