Error when opening Modern UI Editor

Hello ​@eleanorp ,

It seems that specific file is either missing from the site directory or the system is struggling to use it. I'd suggest restarting the application via the Apply Updates screen first; if that doesn't help, contact Acumatica support.

@eleanorp Valentyn's right that it's a missing file. The file belongs to the site, not to your customization projects, which is why you get the error on every screen in every project.

MYOB's Acumatica knowledge base lists this exact error as a known issue, and the workaround they give is to contact support:
https://enterprisesupport.myob.com/knowledge/on-publish-customizations-clicking-on-modern-ui-editor-throws-an-error-failed-to-run-load-modules-for-html-merge-loader.js-on-upgrade-sandbox-or-some-live-sites
Their page marks it fixed in 2026 R1, but you're on 26.101.0225.2 and still hitting it.

The Modern UI Editor merges each screen's HTML with your customizations by running a Node.js script on the server, App_Data\HtmlMerger\package\html-merge-loader.js. When that file isn't there, the editor first runs Node to unpack it from FrontendSources\dist\packages\build-tools.tgz. Your error means that unpack failed. It never gets as far as loading the HTML, which is why the HTML tab in your screenshot is empty.

If Acumatica hosts your sandbox, open a case and send them those two paths along with the MYOB link. If the site is on your own server, Node's own error is saved to App_Data\logs\node-<date-time>-loader.js-error.txt in the site folder.