If you've ever created a campaign, you may have noticed a red banner at the top of your donation page that says:

This appears primarily when you have hidden/turned OFF this campaign from your default giving form.

Simply turn ON the campaign by clicking the 3 stacked dots under "Actions" to the right of your default giving form here > Edit > Donation > scroll down to the Campaigns section > toggle ON the campaign you want to appear > Save.

Give your screen a hard refresh (CTRL + F5) if you don't see the update within a couple of minutes.