A technical problem with the server created an error. Try again to continue what you were doing
解决办法：
In vendor/magento/module-theme/view/adminhtml/ui_component/design_config_form.xml file change fileUploader to imageUploader on line number 57 and 154
在根文件夹：
vendor/magento/module-theme/view/adminhtml/ui_component/ 下面找到 design_config_form.xml ，复制这个文件备份
cp design_config_form.xml design_config_form.xml.back编辑design_config_form.xml的57行和154行 ,将 fileUploader 改成 imageUploader，保存
部署静态内容，并完成了重新索引和缓存清除
php bin/magento setup:static-content:deploy -f
php bin/magento indexer:reindex
php bin/magento cache:clean
php bin/magento cache:flush