App Store not accepting my password -- OS 10.13.6

Are you looking for your login user name to your iMac?

Your Mac user account is your login user name to your iMac

See the following Apple articles:

If you forgot your Mac login password -> https://support.apple.com/en-us/HT202860

Change the login password on Mac -> https://support.apple.com/en-gb/guide/mac-help/mchlp1550/mac

Reset your Mac login password -> https://support.apple.com/en-mn/guide/mac-help/mh35902/mac

If you can't reset your Mac login password -> https://support.apple.com/en-gb/HT212190

See also -> Ways to remember passwords on Mac

Ways to remember passwords on Mac – Apple Support (UK)

It's the one for the Apple App Store that is the problem

That is your Mac user account is your login user name to your iMac.

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Your Apple ID password is the password you use to sign in here.

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.

See the following Apple articles

Sign in with your Apple ID -> Sign in with your Apple ID - Apple Support

Manage and use your Apple ID -> Manage and use your Apple ID – Apple Support (UK)

If you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.