Same problem on an other site migrating J3S3 ->J4S4.0.6 : not possible to create content.
In this cas, the code for the button add content is :
<button data-bs-toggle="modal" data-bs-target="#collapseModal3">
But the code for the seblod content menu, not displayed, is in a this div :
<div id="collapseModal2">
So #collapseModal3 doesn't match with <div id="collapseModal2">