Error 'Can't Upload' on attempt to attach or upload Files on a record

Review your Content Version custom field's attributes to see if any fields are currently universally required

1. Navigate to Content Version from Setup

2. Check the required fields on the list.

Open each field in the "Content Version Custom Fields & Relationships" section by clicking their Field Label

• If a field has the "Required: Always require a value in this field in order to save a record" checkbox selected, uncheck it, and save. If your fields need to be required consider implementing the field requirement on the desired Content Version Page Layout(s) instead. See Customize Page Layouts with the Enhanced Page Layout Editor for more details.  

• Alternatively, if your custom field is a picklist and you're utilizing record types, you may also set a default value for the field as a workaround. See Edit Picklists for Record Types and Business Processes for more details.

Content Version Validation Rules could also generate the same error. Navigate back to Salesforce Files (Classic) or Content Version (Lightning) as described above and click Validation Rules. Temporarily deactivate the validation rule and/or modify it.
 

Notes:

• As per the Files and Content: What’s Different or Not Available in Lightning Experience documentation, "In Lightning Experience, uploads to the Notes & Attachments related list are files."  

• Files utilize and share the same architecture as Content. When uploading a File via either the "Upload Files" button in the Notes & Attachments related list or via the "Add Files" button in the Files related list a related Content Version record is created.

• External (Community/Partner) users can upload the files to the records they have access to or it can be private if they simply upload files in the Files component. They don't get access to upload files in other libraries available on the site. 

• Since there isn't a way to specify or set the Content Version record's universally required field's values when uploading a File via related lists it causes the File upload to fail.  

• If none of the proposed solutions allow for file upload, please create a Support case.  

If Apex Triggers exist on Content objects, troubleshoot in a sandbox by marking them inactive to test whether the error is still present.

• See Disable a Trigger in a production environment for more details.  

• Check the File Storage and make sure that they did not exceed their limit (must be within 100% or below). Review Monitor Data and Storage Resources.

• If they are beyond the limit, advise to delete some of their Files or to request to increase their sandbox File Storage. 

• Kindly verify if there are any triggers associated with ContentVersion in the validation type deployment, as the deployment during regular working hours may cause a temporary disruption.

Related Issue

If you are experiencing the following behavior, please log a case with Salesforce Support to enable "Content Delivery Setup."

• In a Private Chatter Group, if a User tries to drag and drop or select file from "Files", they see "Can't upload <Filename>" error message.

• The same end user is able to upload file by uploading via Chatter Publisher.