Dynamodb PutItem operation with conditional expression

Context

Have a DDB table with schema:

HashKey: UserId
SortKey: UUID
Attribute: status

Question

Want to compose a putItem request with below condition:

1. If userId does not exist, write to the table with PENDING status
2. If userId already exist, only allow write if the existing record is NOT in PENDING status. In another word, if the existing record is in SUCCESS status, allow write to the table.

Expectation

Write Request: Andy(UserId) 123(UUID) PENDING Only allow write success if there no existing Andy(UserId) PENDING in the table already, that is, at any time, the table should only have one record with Andy(UserId) PENDING.

What I have tried

First try:

Expression.builder()
   .expression("#status <> :pendingStatus")
   .putExpressionName("#status", "status")
   .putExpressionValue(":pendingStatus", AttributeValue.builder().s("PENDING").build())
   .build();

Does not work as expected: if Andy(UserId) 123(UUID) PENDING already exist in the table, and I want to write Andy(UserId) 456(UUID) PENDING, the operation should fail with conditional check failure, but it was succeeding.

Second try:

Expression.builder()
   .expression("(attribute_exists(userId) AND #status <> :pendingStatus)")
   .putExpressionName("#transactionStatus", "transactionStatus")
   .putExpressionValue(":pendingStatus",
                       AttributeValue.builder()
                                     .s("PENDING")
                                     .build())
   .build();

Does not work as expected: This condition does not allow new userId to be inserted into the table.

Third try:

Expression.builder()
   .expression("(attribute_exists(userId) AND #status <> :pendingStatus) OR attribute_not_exists(userId)")
   .putExpressionName("#transactionStatus", "transactionStatus")
   .putExpressionValue(":pendingStatus",
                       AttributeValue.builder()
                                     .s("PENDING")
                                     .build())
   .build();

Does not work as expected: if Andy(UserId) 123(UUID) PENDING already exist in the table, and I want to write Andy(UserId) 456(UUID) PENDING, the operation should fail with conditional check failure, but it was succeeding.

I think either I have to change change the table key schema or I need a working condition expression to achieve this, any help would be appreciated.