Many times in different versions of WordPress, one may encounter problems locating the already installed classic editor. A bit of searching around may be needed and will be well worth. Many WordPress enthusiasts prefer to use the classic editor over the block editors. So typical blues like locating the classic editor are normal. Here is an answer.
Each version of wordpress or different users may need different playing around finding an unlocatable Classic editor but a persistant hide and seek will surely lead you to the answer.