I use the 100GB plan, but only 2GB is visible in the cloud

Can you please include the wording of the warning message that you are receiving?  Would be easier for us if you could write the text in this response, for faster translation.  Also, can you confirm that you do not have any very large files (would probably videos) that could run over the number in your upload?

Thanks,

Peter

The top of the picture is the number of photos of the mobile LR (4054) and the bottom is the number of photos of the web LR (3428).
And a warning message appears stating that synchronization is not possible due to insufficient cloud capacity.

From this I can see that the 100 Gb capacity was not given.

Then the cloud should be able to use all 100GB of LR for synchronization. However, only part of it is synchronized and it is not synchronized with a message that the capacity is insufficient. This does not explain

You have purchased the 100Gb Lightroom Mobile plan from a third party site.  This entitles you to storage for Lightroom Mobile only, not for other Creative Cloud applications or content.  To access your Lightroom photos on the web, login to lightroom.adobe.com.  

- Peter