How to cancel subscriptions in my other Apple account which is not accessible anymore?

You can only do it by getting access to the account. You do not, however, say why you can't get into the account.

This article tells you the various ways you can try to discover your forgotten Apple ID --> If you forgot your Apple Account primary email address or phone number - Apple Support Typically an Apple ID is an email address you have used. If you are currently using a mobile device that is logged into your account, go to Settings and tap your name at the top. Look in Keychain, and in a browser's security settings if you use it to automatically sign into Apple when using a browser.

This article outlines what to do If you forgot your Apple ID password --> If you forgot your Apple Account password - Apple Support

or go directly to this link --> http://iforgot.apple.com/

If you find you can’t sign in, reset your password, or receive verification codes, you can request account recovery (read this support article: How to use account recovery when you can’t reset your Apple Account password - Apple Support ) to regain access to your account. Account recovery might take a few days or longer, depending on the specific account information you can provide to verify your identity. More detail in —> Activation Lockout Issues - Apple Community

If you still need help this page provides country-specific Apple Support contact information ➞ Contact Apple for support and service - Apple Support

Your alternative is to default on payment with the card used in that account by canceling your card. This is kind of the sledgehammer approach to dealing with the issue and is not one you should use unless you have no alternative. It could possibly result in the account never being usable to you again, plus it will affect other payments being made using that card.

If you cannot access it anymore you first need to get access to It in order to end the subscription

Try to reset your password at iforgot.apple.com