In some cases we tried to edit the page and tried to add web
part click on Add a Web Part link but in the top ribbon we can able to see only
Apps inside the categories, we verified the permissions for that particular user
having the Full Control to the site.
Resolution:
If we are currently using sub site in this case, we don’t have permissions to
access the top level site (site collection), at least the users having read
access to the top level site (site collection) or at least read permissions to
the web part gallery available at top level site (site collection).
Once we granted the read permissions to the users to the
root site (site collection) or web part gallery then only users can able to add
the web part to their site with all available web parts, because the web part
gallery is available only at top level site collection only and they can able
to see all the web part categories without any problems.