If the automatic installation fails, you can always manually copy the rig .blend files to Auto-Rig Pro. Please follow this tutorial on YouTube, or follow the steps below:

1. First, make sure you have installed Auto-Rig Pro
2. Download the auto_rig_pro_rig_library .zip file from Blender Market to your computer
3. Unzip the .zip file -> open the armature_presets folder -> select all the .blend files and copy them
4. In Blender go to: Edit -> Preferences -> Add-ons -> expand Auto-Rig Pro -> under Paths: look for the Armatures file path
5. Navigate to this file path on your computer -> paste all the .blend files on this location
6. Restart Blender
7. Go to the Auto-Rig Pro addon -> open the Add Armature dropdown menu -> notice the rigs have been added

Auto-Rig Pro 3.69.10 and up.

Please follow this tutorial on YouTube, or follow the steps below:

1. First, make sure you have installed Auto-Rig Pro
2. Download the auto_rig_pro_rig_library .zip file from Blender Market to your computer
3. In Blender go to: Edit -> Preferences -> Get Extensions
4. Open the dropdown menu in the topright corner -> Install From Disk -> browse to the auto_rig_pro_rig_library .zip file on your computer
5. Restart Blender
6. Go to the Auto-Rig Pro addon -> open the Add Armature dropdown menu -> notice the rigs have been added

Please have a look at this YouTube tutorial I made.

First of all, for both methods make sure that in the Outliner your model is placed within the root collection of your rig.

• Appending (creating a copy of the rig):
  File > Append > browse to the rig file on your computer and double-click it > Collection > select the root collection of the rig (the one with only the name of the rig, so NOT the collections rigname_cs and rigname_grp_rig) > Append.
  
  
• Linking (creating a reference to the rig source file):
  - File > Link > browse to the rig file on your computer and double-click it > Collection > select the root collection of the rig (the one with only the name of the rig, so NOT the collections rigname_cs and rigname_grp_rig) > Link
  - Object Mode > Object > Relations > Make Library Override
  - Note that you can’t edit a linked rig. You will need to do that in the rig source file.

Please be aware that in Blender a rig can reference only one action. So when you bring in a rig (through appending or linking) that contains multiple animations,  it will only contain the action that is active in the original rig file.

This happens when you change the amount of bones in a limb to a SMALLER amount of bones. The channels of the bones that have been removed will still be present in the Graph Editor and give an error. You can simply select all these channels and delete them.

1. Allosaur - walk
2. Apatosaurus - walk
3. Bear - walk
4. Bird - fly
5. Cat - walk
6. Cow - walk
7. Crab - walk
8. Dog - idle, trot, run, dig
9. Dolphin - swim
10. Dragon - fly
11. Duck - walk
12. Elephant - walk
13. Giraffe - walk
14. Gorilla - walk
15. Horse - gallop
16. Human - walk, run
17. Kangaroo - jump
18. Lizard - walk
19. Mouse - run
20. Ostrich - run
21. Pig - trot
22. Shark - swim
23. Snake - slither
24. Spider - walk
25. Turtle - walk

Because all the rigs have been setup with export to a game engine (Unreal/Unity) in mind. A frame rate of 30fps is the standard for game animation.

Stretchy arms/legs are turned off by default in order for the rigs to be game engine compatible. 

But these are the steps to turn on stretchy arms/legs:

• Stretchy FK arms/legs
  Pose Mode > select the hand/foot controller > Tool tab > adjust the Stretch Length (FK) value.
  
  
• Stretchy IK arms/legs
  Pose Mode > select the hand/foot controller > Tool tab > adjust the Stretch Length (IK) value.
  
  Alternatively, you can turn on the Auto Stretch property. When you move the hand/foot IK controller, the arm/leg will automatically stretch.

Make sure to turn on the Fake User shield for all the actions of a rig.
When you don't turn on Fake User for the actions, it can happen that Blender will remove them when re-opening the .blend file.