Possible root cause:
1. Your license doesn't have the project mapping module.
2. Project mapping module is not enabled.
3. Your General Ledger Account Receivable doesn't have a Class value.
4. The Class value is not numerical. It should always be numerical.
5. Project doesn't exist in Acomba.
6. You're trying to map not a receivable account but an account of another type. This module works only for receivable accounts (AR).