This issue can be caused by a number of things but is rarely theme-related. It is often due to an issue with web hosting such as running out of storage space, temporary upload space or with file permissions but it may also be caused by a plugin or child theme. We recommend trying these things.
1. Try deactivating all plugins then go to Media and try uploading a file. If the problem goes away while all plugins are inactive, then reactivate them one by one to determine which is causing the problem.
2. Go to Tools > Site Health and report any errors to your web host. Try uploading after any issues with Site Health are resolved.
3. Read How to Fix the WordPress HTTP Error for more things to try. You can use the WordPress Support Forum to search for answers or post a question of your own.