AMPScript does not look to be working

Question

I have a column called First Name in Data Extension TriggeredSendDataExtension - 20160917_111231. I am trying to print FromEmail from the data extension row by passing the first name as the send time parameter. But it does not looks to be working. I am keeping this in Email Template.

Set @FromEmail = Lookup("TriggeredSendDataExtension - 20160917_111231", "FromEmail", "First Name", %%First Name%%)

%%=v(@FromEmail )=%%

Here 
dataExtension - TriggeredSendDataExtension - 20160917_111231
returnColumn - FromEmail
matchColumn - First Name
value - %%First Name%%

Also I have tried the below pattern. It is also does not seem to be working.

  %%[ 
 SET @MYSUB= %%Subject%%
 Set @Subject= Lookup("TriggeredSendDataExtension - 20160917_111231", "FromEmail", "Subject", @MYSUB) ]%%

    %%=v(@Subject)=%%

With this I am getting

Error 1: Script SET Statement Invalid
An error occurred when attempting to resolve a script expression. See inner exception for detail.
Script Expression: %%Subject%%
MemberID: 7273985
JobID: 0

How do I resolve the issue?

Answer 1

You can not pass %%First Name%% directly in any LookUp function.

I would suggest you to use AttributeValue AmpSCript function and initialize First Name in variable.

%%[
SET @FirstName = AttributeValue("First Name")
SET @FromEmail = Lookup("TriggeredSendDataExtension - 20160917_111231", "FromEmail", "First Name", @FirstName)
]%%

%%=v(@FromEmail)=%%