It would not be correct to use the indicative mood in this case.
Agradecer is a 'Wish' verb, one of six classes of verbs that trigger the use of the subjunctive mood; a discussion of the WEIRDO verbs can be found
here (locate the 'Words and Phrases that Trigger the Subjunctive or Indicative' subheading on their page and click on each of the six links, beginning with 'Wishes'). You can find more sites by doing this search: Spanish WEIRDO agradecer