To have access to your variables between two functions, you have to define them in an outer scope. In your case _endOutLoc, _startOutLoc, _rootLoc are defined locally to ProxySkel function and they are not visible to jointsCreate function In additional to that, there are several other mistakes

• as it was mentioned in the comments to your question, _rootLoc have to be passed with index 0, because it stores value as a list, not as a string
• if you are using parent constraint for the alignment, I would guess you want to delete them once the alignment is finished
• there is no need to call parent command for the joints, as they are already parented in the order of cmds.joint calls. Here is a note from documentation "In the creation mode, a new joint will be created as a child of a selected transform or starts a hierarchy by itself if no transform is selected." if you don't want such behaviour, you should clear the selection
• cmds.window('window1',menuBar=1) have to be always called, not inside the existance window condition, otherwise the UI elements will be attached to the script editor window

here is an updated script

from maya import cmds

global _endOutLoc
global _startOutLoc
global _rootLoc

def proxySkel (self):
    global _endOutLoc, _startOutLoc, _rootLoc
    
    target = cmds.ls (selection = True)
    DDD = cmds.textField (textNam,
                          query = True,
                          text = True)
                          
    # CREATE THE LOCATOR 
    _endOutLoc = cmds.spaceLocator (position = (0, 0, 0), name = DDD + '_locEndOut')
    _startOutLoc = cmds.spaceLocator (position = (0, 0, 0), name = DDD + '_locStartOut')
    _rootLoc = cmds.spaceLocator (position = (0, 0, 0), name = DDD + '_locRoot')                          

    # MOVE END LOCATOR IN SPACE
    cmds.move (5, 0, 3, _endOutLoc, absolute = True)
    cmds.move (0, 0, 3, _startOutLoc, absolute = True)

    return _endOutLoc, _startOutLoc, _rootLoc    


def jointsCreate (self):
    global _endOutLoc, _startOutLoc, _rootLoc
    
    if None in [_endOutLoc, _startOutLoc, _rootLoc]:
        print('locators are not created yet')
        return
    
    target = cmds.ls (selection = True)
    DDD = cmds.textField (textNam,
                          query = True,
                          text = True)
    
    # CREATE THE JOINTS 
    rootJnt = cmds.joint (position = (0.0, 0.0, 0.0), name = _rootLoc[0] + '_jnt')
    endOutJnt = cmds.joint (position = (0.0, 0.0, 0.0), name = _endOutLoc[0] + '_jnt')
    startOutJnt = cmds.joint (position = (0.0, 0.0, 0.0), name = _startOutLoc[0] + '_jnt')
    
    # PLACE THE JOINTS INTO SPACE
    rootConst = cmds.parentConstraint (_rootLoc, rootJnt, mo = False)
    endOutConst = cmds.parentConstraint (_endOutLoc, endOutJnt, mo = False)
    startInConst = cmds.parentConstraint (_startOutLoc, startOutJnt, mo = False)
    
    # DELETE CONSTRAINTS AFTER ALIGNMENT
    cmds.delete (rootConst, endOutConst, startInConst)
    
    # CREATE THE JOINT HIERARCHY
    #cmds.parent (endOutJnt, rootJnt)
    #cmds.parent (startOutJnt, endOutJnt)




if(cmds.window('window1',q=1,ex=1)):
    cmds.deleteUI('window1')
    
cmds.window('window1',menuBar=1)

cmds.columnLayout()

textNam = cmds.textField(width=150,
               text='Name',
               backgroundColor=[0.6233005264362554, 1.0, 0.9765011062790875])

cmds.separator()

cmds.rowLayout(numberOfColumns=3)

cmds.button(width=65,
            command = proxySkel,
            backgroundColor=[0.0, 0.2187533379110399, 1.0],
            label='Locators')
            
cmds.separator(style='none',
               width=3)
               
cmds.button(width=78,
            command = jointsCreate,
            backgroundColor=[0.7971007858396277, 0.0, 0.0],
            label='Joints')
            
cmds.showWindow('window1')