Hi,
this is most likely caused by code that is not compatible with the latest WordPress release (5.5 and up). The error should indicate the source, which could point to a specific plugin for instance. Make sure that you have updated all plugins to the latest versions.
As a temporary workaround, you could install and activate the jquery-migrate-helper plugin (https://wordpress.org/plugins/enable-jquery-migrate-helper/), which should resolve the issue (but not the incompatibility, therefor I call it “temporary”.
