Can't use basic step "Then" in package

[QAPurporse]

Hello,

Looking for help, can't use one of basic step "Then" from package:
Then I can see the 'New' command

I received such error:
error: Could not load type 'FluentAssertions.Collections.SelfReferencingCollectionAssertions`2' from assembly 'FluentAssertions, Version=6.11.0.0, Culture=neutral, PublicKeyToken=33f2691a05b67b6a'. (0,0s)

I take a look and not see this type in package Fluent Assertion.

What else it could be ?