Having problem to writing a SQL query

What I've done:

USE [cpqaAnswers]
GO

CREATE TABLE [cpqaAnswers].[cpqa].[tbl_SK_typSubmit_01](
	[Id] INT,
		[SubmissionDate] DATE,
			[SubmissionType] NVARCHAR(256),
				[Amount] DECIMAL(10,2),
					[PaidAmount] DECIMAL(10,2),
						[Unique_Id] NVARCHAR(27)
					)

USE [cpqaAnswers]
GO

BULK INSERT [cpqaAnswers].[cpqa].[tbl_sk_typSubmit_01]
	FROM 'C:\Users\ks\data.txt'

data.txt content:

/*

Id    SubmissionDate    SubmissionType      Amount     PaidAmount   Unique_Id
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1      2020-01-04       Submission          1166.00    562.00       test123
2      2020-06-01       Submission         -1166.00   -562.00       test123
3      2020-06-01       Correction          1166.00    579.00       test123
4      2020-11-02       Internal Complaint   587.00      0.00       test123

*/

Check ...

USE [cpqaAnswers]
GO

SELECT [Id]
      ,[SubmissionDate]
      ,[Amount]
      ,[PaidAmount]
      ,[Unique_Id]
  FROM [cpqa].[tbl_sk_typSubmit_01]
GO

Id	SubmissionDate	Amount	PaidAmount	Unique_Id
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1	2020-01-04	    1166.00	562.00	    test123
2	2020-06-01	   -1166.00	-562.00	    test123
3	2020-06-01	    1166.00	579.00	    test123
4	2020-11-02	    587.00	0.00	    test123

Poster's query:

USE [cpqaAnswers]
GO

Select Count(Id) As [Count], Sum(PaidAmount) As [PaidAmount], Max(Case When Amount <=0 then 0 else Id end) As [LastId]
	from [cpqa].[tbl_sk_typSubmit_01] 
		Group by unique_Id

What I get as a return:

Count	PaidAmount	LastId
~~~~~~~~~~~~~~~~~~~~~~~~~~
4	    579.00	    4 

And although you state that the desired output under [LastId] is "3", it never will be "3", it will always be the last row of data [LastId] because the conditional "<=0 THEN 0 ELSE Id END" (as you can see in the data) IS NEVER "0"; the last row "4".

So you've got youself a Type II error in you reasoning.