Can’t dismiss follower

Are you on the Companions quest chain by chance? Possibly the quest Proving Honor? Starting this quest (and I think several other Companions quests) sets Farkas (in this case) as your Follower. If thats the case, he's waiting for you at Dustman's Cairn and you won't be able to dismiss him until you complete the quest.

The mod, Better Dialogue Controls (original / Legendary Edition) | (Special Edition), is an attempt to fix this.

Description:

This mod will be most useful for people who like to use both the keyboard and mouse, it fixes issues where the mouse just flat out prevents some keyboard controls from working as expected. If you exclusively use the mouse to scroll and click through dialogues you may not have run into some of these issues. Likewise, if you do not move the mouse at all after entering a dialogue. If you move the mouse pointer after entering a dialogue, then you will run into the issues below:

• A fairly common occurrence is when you leave the mouse pointer somewhere over the menu and you want to use the keyboard controls. If you like to switch to the keyboard navigation now and then, you will run into issues where the mouse pointer just flat out prevents you from scrolling to the top or bottom of the list with the keyboard controls.
• Another common occurrence is that as you enter the dialogue, you think the first item, next to the pointy bit, is the selected item. However when you press E, you find that the 2nd or 3rd item has been selected. This happens because the mouse pointer takes the focus away from the default option. Often times, I had to move the mouse pointer out of the way when entering the dialogues because of this.

Solution:

• The mouse focus now highlights items, but does not make them active. Thus the active item is always at the center of the list (next to the arrow/pointy bit). When you enter a dialogue, the mouse pointer never takes the focus away from the default option.
• The keyboard controls lets you scroll all the way to the first or last item, no matter if the mouse pointer is hanging over the menu.
• Scrolling the list with the mousewheel, or up/down controls always maintains the center item as the active one.
• You can still click off center dialogue items with the mouse, the mouse just won’t affect the menu until you click.
• The mousewheel scrolling has a smoother transition. It happens because I changed the mousewheel scrolling to work exactly as if you pressed the up/down controls. I didn’t plan to change the mouse wheel scrolling appearance, but I thought it was nice so left it in.

^{From: https://www.nexusmods.com/skyrim/mods/27371}