This error is being shown if the Class field is set as Mandatory on the Journal Entry form:
USER_ERROR: Please enter value(s) for: Class
Turn off the Make Classes Mandatory preference.
1. Navigate to Setup > Accounting > Accounting Preferences.
2. Under the General tab, uncheck the Make Departments Mandatory box.
3. Click Save.
Note: This preference affects not just Journal Entries but other transactions as well. This can be turned back on after doing the Asset Proposal.
Customize the Journal Entry form to make the Class field non-mandatory
1. Navigate to Customization > Forms > Transaction Forms.
2. Click the Edit link besides the form with the Type of Journal and with the Preferred box checked.
3. Click the Screen Fields tab > Main sub tab.
4. Uncheck the Mandatory box for Class.
5. Click Save.