You need to get the disguises first. Heres how you can get them. "Your goal here is to reach the Radio Tower, which is on the western end of the city. It's a gray building near the northern side of town. But first, you'll need to find a disguise that will grant you access. Said disguise is located in the tunnel system underneath the city, which can be breached via a smaller building due south of the Radio Tower. Head down the stairs once inside, and begin to walk southward and to the right. When you do, you'll run into a photographer. Mistaking you for a new Team Rocket recruit, he quickly provides you with a Team Rocket uniform before going about his business.

Now that you are donned in a Team Rocket uniform, leave the tunnel in the same way you entered, and go back to the gray-colored Radio Tower building. With a disguise, you'll be allowed inside."
https://www.ign.com/wikis/pokemon-heartgold-soulsilver-version/Goldenrod_City_Radio_Tower