Cannot access Finder Tags

One test is to open the Spotlight 🔍 tool from the menubar and enter a full or partial case-insensitive tag name string substituting for xxx. The second form is if you are in a Germanic language country where "tag" can have several different meanings:

tag:xxx
kMDItemUserTags:xxx

If this finds any of your custom-tagged filenames, then you know Spotlight is indexing tags properly.

When you arrow down in the Spotlight interface to a found file, you can do the following:

• Press the ⌘-key to see the path of the file on the same entry
• Press ⌘R to open a Finder window with the file selected
• Press return to open the file in its default application

If no tags are found above:

Check the Finder Settings > Tags panel to ensure that your custom tag names all have a blue checkmark to indicate they are Finder active.

In System Settings > Siri & Spotlight, select every Search Results category except Siri Suggestions. Click the Spotlight Privacy… button on that panel, and drag and drop your Desktop Macintosh HD folder icon into the now open Privacy panel. Wait 10 seconds and then select that Macintosh HD entry and click the [-] button. This will trigger Spotlight to re-index the drive and it may take several minutes to complete.

You can verify indexing in progress by opening that Spotlight menubar item again and you should see an oscillating indicator:

I never check tags on a file from the Finder sidebar. Ensure that in the Finder Settings > Tags panel, the tags are all marked active ☑︎ and not -⃞ or empty. These tag names must be active in the preceding Tags panel or they will not be indexed by Spotlight or appear in the Finder sidebar Tags categories.

Finder must activate those custom tags in its Tags settings panel. Spotlight must index those files bearing your custom tags and if they are on an external drive that has not been indexed, they will not be locatable via the Finder sidebar or Spotlight (e.g. tag:red).

You are welcome.