As far as I know there aren’t any compatibility issues with 5.6, so it may be something else.
What version of MapPress are you using? Please be sure it’s the latest version.
Conflicts with other plugins are a major cause of errors. To check: please deactivate ALL other plugins and switch to the twenty twenty theme, for a moment. Did the maps work? If so, reactivate one by one to find the conflict.
If not, please use the contact form to send a login so I can see the problem in the admin.