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1.
The message is clear:
Just do it.
You need to have a payment method on file, even if the app you want to download is free
Check which payment method you have on file
See also ⤵️
Payment methods you can use with your Apple ID
Payment methods you can use with your Apple ID – Apple Support (UK)
Add money to your Apple ID balance
Add money to your Apple ID balance – Apple Support (UK)
2.
Check your Password.
See -> If you can't connect or sign in to iCloud
If you can't connect or sign in to iCloud - Apple Support
If you forgot your Apple ID password
If you forgot your Apple ID password - Apple Support
Then see this Apple article: What to do after you change your Apple ID or password
What to do after you change your Apple ID or password - Apple Support
You are sign in here . . . with your Apple ID & Password.
Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.
You can’t merge multiple Apple ID accounts.
If necessary, click here for more support and assistance.