Can I use SQL Server Standard Edition as Primary and Enterprise SQL Server as Secondary node? I have Software Assurance for SQL Server Enterprise Edition and I need to setup High Availability on it.
The issue is my primary SQL Server has a Standard license.
Could I add SQL Server Enterprise edition as secondary replica?
No, this is not possible. Even if you'd manage to do such an installation, it would not be supported by Microsoft.
The documentation explicitly states that the editions must match:

The SQL Server edition you are installing must match across all the
nodes in a SQL Server failover cluster instance. When you add a new
node to an existing SQL Server failover cluster instance, make sure
that you specify that the edition matches the edition of the existing
failover cluster instance.

Although licensing questions are off-topic, a warning about your purposed installation is in order. In case of a licensing audit, the auditor would be quite likely to accuse you of running unlicensed setup. Licenses for Enterprise Edition are not cheap. You don't really have much bargaining power when license auditor has proof that you are not compliant.