2 answers
Seems you are trying to pre-populate a field while creating a record.To troubleshoot this, a good place to start would be - Log in to Community and click the standard New button, select record type and note down the URL that you have now (on edit page).Compare it to the one you have set up in your custom button. URL may be different for Community and standard Salesforce or you may also want to verify that the tab a0A is included in the Community and is not hidden for the user (should be default On or Off).