How can I merge two apple Id's into one?

You might want to start your own post. A new post would be much more visible. 

What happens when you try to log in? Error message(s)?

Are you running any anti - virus programs? Using a VPN?

Try setting up another admin user account in System Preferences/Users & Groups to see if the same problem continues. Please post back on whether or not this worked. Also try the Safe Mode. Please post back on whether or not this worked.

Isolating an issue by using another user account 

Safe Mode - About

If it works in the Safe Mode, try running this program when booted normally and then copy and paste the output in a reply. The program was created by Etresoft, a frequent contributor.  Please use copy and paste as screen shots can be hard to read. Click “Share Report” button in the toolbar, select “Copy to Clipboard” and then paste into a reply. This will show what is running on your computer. No personal information is shown. If the log won’t post, try posting it in Pastebin and provide a link in a reply. After pasting the report in a PasteBin page, go to the top of the page, and copy the address in the URL bar. Paste that in a new reply.        Pastebin

Etrecheck – System Information

You can't merge Apple IDs. You can try setting up Family Sharing between the 2 IDs if you want to be able to share purchases made with the IDs.

Family Sharing

Family Sharing - Set up

hi, my Apple ID in my Mac book's App Store does not work