The new Gutenberg editor may not play nicely with all themes.
Therefore, you may want to deactivate the editor. To deactivate, use the Classic Editor plugin.
Alternatively, you can use both the classic and the Gutenberg editors. Install the Classic Editor plugin and amend the setting:
“Use the Gutenberg editor by default and include optional links back to the Classic editor”
As a result, you can use and learn the new Gutenberg editor while reverting to the Classic editor at any time.