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.