Possible reasons:

• amount must be positive
• unknown asset_code
• a base asset must have a Base fee or a non_base asset must have a NonBase fee, but not any other combination
• could not parse fee_amount
• fee_amount must be greater than or equal to zero
• transaction with this external_id was already attempted

Possible reasons:

• failed to get asset code
• failed to get authority public key
• attempt to verify existing contract failed
• failed to get treasury user
• treasury user is not configured
• failed to build request data
• failed to look up recipient user
• failed to get recipient user vault
• database error
• could not begin transactional query
• failed to save contract
• failed to save contract actions
• failed to create transaction
• failed to commit database transaction