I am building a Sharepoint site starting from a Communication site. I'd like to have some classic pages on this site, and have figured out how to create one, but when I try to add a web part to my classic page, there are none available. I have the ability to upload web parts. I just need to know where to find the standard set of classic web parts so that I can upload them. Are the classic web parts available from Microsoft somewhere? I can't find them but can only find custom web parts available from others.
Go to: https://Your-Base-URL/_layouts/15/newdwp.aspx
Go to: https://Your-Base-URL/sites/Sub-Site-URL-Slug/_layouts/15/newdwp.aspx
On this page, you can select any of the OOB webparts to populate the gallery, just by selecting them and clicking the 'Populate Gallery' button. (see image below)
From one of the above URLs (insert the proper URL elements in bold in the above URL examples) you can enable items in the webpart gallery for the relevant site by checking off the webparts you want enabled and clicking the ‘Populate Gallery’ button.
Also, try checking it immediately after performing the above, it sometimes works instantly, otherwise give it a couple of hours (it should not take a whole day - but might) for the classic webparts to appear in the gallery on classic pages.
Also, I think it would be helpful to other users who may come upon this question if you actually posted with your description as to how you enabled classic site pages on a Modern SP site.
Please mark this as the answer of this Answered your question.