Error about Resolving variable '$uri["aaa"] failed: TypeError: string indices must be integers

i have a problem with use variable in yaml file

my variable is type string but execute is have type error:string indices must be integers.

this is my yaml file
uri:
aaa: “/api/aa”

my code robot
*** Test Cases ***
TC001
Request Withdraw

*** Keywords ***
1.
Request Withdraw
${response}= POST On Session ${SESSION_NAME} ${uri[“aaa”]} headers=${header} json=${request_withdraw} expected_status=200

execute is fail

but I use
2.
${response}= POST On Session ${SESSION_NAME} /api/aa headers=${header} json=${request_withdraw} expected_status=200

I can execute is pass

I don’t understand why i can’t write no.1

I can solve the problem. because I declare a duplicate variable