Unable to use "Move to Top" UI Action on the Sprint Planning tab of the Agile Board

You may be having this issue due to some of your Global Rank field values for Stories not being populated.
Please take a look at the following Community post, particularly point 2.

https://community.servicenow.com/community?id=community_article&sys_id=aa622adfdb771b000be6a345ca96195e

This will provide you with the steps that you will need to take to resolve this issue.

NEXT STEPS:

Running the following Script in System Definition > Scripts - Background should clear up the issue

var gr = new GlideRecord('rm_story');
gr.addNullQuery('global_rank');
gr.query();

var rankProcessor = new AgileRankProcessor();
while (gr.next()) {
rankProcessor.generateNewIndex(gr);
gr.setWorkflow(false);
gr.update();
}

Make sure to test in a Sub Prod first!