In the Space class in spaces.py in version 0.7.4 (line here) - skopt expects scikit-learn to return a tuple for version - it now returns a Version class in scikit-learn 0.23.2.

I see that this check is no longer present on the master branch, so I haven't submitted a pull request, a new release would fix this bug.

I'm sure you know this already, but sklearn has its own parse_version function, which would return a compatible comparison object. 😄