JQ error when trying to parse JSON string - unexpected '['

When trying to query a JSON str, using the script below, I get an error.

db_secret_data=$(aws secretsmanager get-secret-value --secret-id "$db_secret_name" --output json)echo "$db_secret_data"db_secret_str=$(echo "$db_secret_data" | jq --arg keyvar "${uid_str}-TY" '.SecretString.[$keyvar]' -r)

Here is the error:

jq: error: syntax error, unexpected '[', expecting FORMAT or QQSTRING_START (Unix shell quoting issues?) at <top-level>, line 1:.SecretString.[$keyvar]   

I've gone over the syntax over and over, and can't see what's wrong. Can anyone help?