What to do about Eclipse's "No repository found containing: ..." error messages?

What I did was:

1. I went to Window > Preferences > Install/Update > Available Software Sites, then for each enabled site I added a / to the end of the URL (if it wasn't there already), then clicked Reload as per @Hunternif answer. But the problem still persisted.
2. Then I disabled all the software sites and re-enalbed them one by one and ran updates to keep only those that worked. After step 2. the problem was solved. Now I have enabled only the update sites that do not give the error and updates work.

None of the other answers worked for me (Windows 10, Eclipse Neon). Running Eclipse as Administrator fixed it.

I've had the same issue since mid 2018. Performing a search, this issue has been reported since 2011. I'm surprised workarounds are proposed for this. Unfortunately they havent't worked for me, currently the only fix seems to completely reinstall Eclipse. As the most upvoted suggestion here, many of the suggestions are contradicting, suggesting trial-error guesses.

Besides these workarounds, IMHO this requires a fix. We're in 2019 now, surely there is a way to fix this recurring issue? How can there not be a simple fix i.e. if repo URL not found: skip and continue with next URL / update (without aborting as critical error and preventing any other update)?

I got this error (September 2020) during an automatic upgrade.

The solution was to add:

http://download.eclipse.org/releases/2020-09

to the list of Available Software Sites.

I also disabled:

http://download.eclipse.org/eclipse/updates/4.15

and several more with release numbers earlier than 4.15, but I don't think that was the important part.

2019 Eclipse/CCS

After trying many of the above techniques described here I noticed this post by Edwin landwealths

The long and short of it is to add the infamous tail "/" and ALSO change "http" to "https". This fixed my issue immediately.

As many already said it is an issue related to the update sites, for me disabling Help → Install new software → “Contact all update sites during install to find required software" fixed the issue for install Sirius. Put for installing epsilon I disable update site as in the picture

I had a few repository URL's saved with HTTPS. I changed them to HTTP and the error was gone.

You can adapt the URL's in the settings: Preferences -> Install/Update -> Available Software Sites

For me I had Spring IDE installed on Eclipse Oxygen. During check for updates it would find updates, then it would fail with the above error.

I then went to the Eclipse Marketplace, clicked on Installed tab and noticed Spring did not come up (it was there before and is currently installed).

Then I searched for Spring IDE (Tools) clicked install, then it notified that some packages were already installed and just needed to be updated.

After completing the install of Spring Tools and restarting, I was able to check for updates and complete the normal software updates.

Install the latest STS, export its site list and import it into yours.

Some of the above solutions worked to resolve some of my errors... it seems that after a while update connections just get corrupted and there is no silver-bullet. Managing updates through the marketplace (Help > Marketplace > 'show updates') allowed me to narrow down the packages with failed dependencies.

Here's what I tried (taken from posts above):

1. switching https to http for software sites
2. running Eclipse as administrator
3. 'disabling' obviously outdated software sites
4. 'enabling' all software sites
5. adding backslashes to the end of software site urls
6. uninstalling and re-installing troublesome software

I was still left with some Mylyn wikitext errors despite trying the suggestions here

Eclipse IDE for JavaScript and Web Developers

Version: 2019-09 R (4.13.0) Build id: 20190917-1200

Issue seems to be with the outdated sites listed in the Window -> Preferences -> Available Software Sites. In this window just update the location of the Buildship site.

For the Buildship, the correct url for different versions can be found from this main site : https://download.eclipse.org/buildship/updates/

For specific versions, navigate further down until you reach the specific version. For example: https://download.eclipse.org/buildship/updates/e416/releases/3.x/

For the latest updates, use url https://download.eclipse.org/buildship/updates/latest/

For those who encounter this problem when installing Eclipse Code Recommenders, you could try the advice from the Eclipse Community Forums:

Go to Help -> Install new software...
Add a repository: 
    Name: Code Recommenders
    Location: https://repo.eclipse.org/content/shadows/releases.unzip/org/eclipse/recommenders/stable/2.5.4/stable-2.5.4.zip-unzip/

I guess this approach can be used for other archived projects, too.

See this may be the issue from the publisher side or may be the URL is inactive

you can install it via install new software feature

If you are stuck with eclipse color theme from marketplace then you just need to replace .com to .io from URL as .com URL doesn't exist

to see how to install properly using this feature check this out

https://itsfoss.com/change-eclipse-color-theme/