How to undo another person's checkout

What version of Kentico are you using and what are the errors you are getting? A global administrator user, I mean user with global administrator privilege level should be able to undo the checkout or at least impersonate as given user and check in that page.

Hi Juraj, our kentico version is V12. The error I'm getting from the Event log is :

Message: Page type with 'cms.wireframe' class name not found.

Exception type: CMS.DocumentEngine.DocumentTypeNotExistsException Stack trace: at CMS.DocumentEngine.TreeNode.get_DataClassInfo() at CMS.DocumentEngine.TreeNode.RefreshNodeDataClassId() at CMS.DocumentEngine.TreeNode.set_NodeClassName(String value) at CMS.DocumentEngine.TreeNode.InitializeClassName(String className) at CMS.DocumentEngine.TreeNode.Initialize(String className, DataRow dr, TreeProvider treeProvider) at CMS.DocumentEngine.DocumentQueryProperties.get_Document() at CMS.DocumentEngine.DocumentQueryBase2.TypeUpdated() at CMS.DataEngine.MultiObjectQueryBase3.CreateQuery(String type) at CMS.DataEngine.MultiQueryBase2.EnsureQuery(String type) at CMS.DataEngine.MultiQueryBase2.EnsureQueries(IEnumerable1 types) at CMS.DataEngine.MultiQueryBase2.Types(String[] types) at CMS.DocumentEngine.Web.UI.CMSAbstractDataProperties.GetQueryInternal(String path, Boolean forceReload, Int32 offset, Int32 maxRecords) at CMS.DocumentEngine.Web.UI.CMSAbstractDataProperties.<>c__DisplayClass53_0.b__0(CacheSettings cs) at CMS.Helpers.CacheHelper.<>c__DisplayClass83_01.b__0(CacheSettings s) at CMS.Helpers.CachedSection1.LoadDataHandled(Func2 loadMethod) at CMS.Helpers.CacheHelper.Cache[TData](Func2 loadMethod, CacheSettings settings) at CMS.DocumentEngine.Web.UI.CMSAbstractDataProperties.GetDataSet(String path, Boolean forceReload, Int32 offset, Int32 maxRecords, Int32& totalRecords) at CMS.DocumentEngine.Web.UI.CMSAbstractDataProperties.LoadData(Object& dataSource, Boolean forceReload, Int32 offset, Int32 maxRecords, Int32& totalRecords) at CMS.DocumentEngine.Web.UI.CMSDocumentsDataSource.GetDataSource(Int32 offset, Int32 maxRecords) at CMS.DocumentEngine.Web.UI.CMSBaseDataSource.get_DataSource() at CMS.DocumentEngine.Web.UI.CMSUniView.ReloadData(Boolean forceReload) at CMS.DocumentEngine.Web.UI.CMSUniView.InitControl(Boolean loadPhase) at CMS.DocumentEngine.Web.UI.BasicUniView.OnLoad(EventArgs e) at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) Machine name:TSTALMWEB19 Event URL:/cms/getdoc/1b87e238-2f73-42e8-a217-e4e15117f073/Home.aspx?viewmode=6&showpanel=1&lang=en-US&langobjectlifetime=request URL referrer:https://tstalm.tst.ubb.com/CMSModules/AdminControls/Pages/UIPage.aspx?elementguid=eeba18a6-c6d0-4c58-a5fa-91e629540986&mode=edit&displaytitle=false&nodeid=1469&culture=en-US User agent:Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/104.0.5112.102 Safari/537.36 Edg/104.0.1293.70

This is the error after I removed the checkout.

That page type was removed in one of the previous versions - it looks like the upgrade was not done right or, this page has been checked out for a long time. I mean, the page in question - what is its type? (Properties -> General) Have you tried impersonating that user?
It might be easier to delete and re-create the page - depending on the amount of content it contains.

Also consider resetting this user password, logging in as this user and checking out. Undo checkout typically could cause issues especially if the page was checked out long ago, cause effectively Undo reverts back to the previous version, and it could be the version before the upgrade as Juraj pointed out.

Therefor typically when user left the company we reset their password and check in all their pages on their behalf.