Try switching to CPU rendering mode and remove unused data by following tutorials like the following and switch to GPU mode again.
https://www.youtube.com/watch?v=i8UYfT4MYKg

If the above solution doesn't work, you are advised to (1) use only the CPU rendering mode, (2) upgrade your graphics card to have more VRAMs, or (3) stay with lower versions that you didn't experience the issue. 

For now, contact "George" at vfxgeorge@gmail.com or admin@digitalmirage3d.co.kr. Start your email as "RE: CITIGEN."

It might take as long as 2-3 business days to get it processed.

It needs to be able to run Blender 2.93 and above. This is a machine-intensive add-on. So you'd better have a high-spec hardware setup. The hardware used to create this add-on was Windows 10, i5-6600 3.30GHz, 32G RAM, RTX3060ti.

* GOOD NEWS: From version 1.6.2 on, even without high-end graphic cards users can now use Citigen in "Proxy View" mode to edit, navigate, and animate the scene. "Proxy Mode" can be briefly turned off to see the real things to be rendered.

* ATTENTION: If you are going to render your projects with your machine only--not using render farms--double-check if you can use GPU rendering option in your machine. As of January 3, 2022, official builds of Blender do not support GPU rendering in MacOS computers, for example. (https://docs.blender.org/manual/en/latest/render/cycles/gpu_rendering.html)

No, it's meant for films or animation in drone-shot-like scenarios. Or you can use your own detailed models for close-up shots with CITIGEN props in the background.

Yes!
In fact, owners of any version of Citigen are shown their respective upgrade prices whenever they visit this (https://blendermarket.com/products/citigen) page again.

• Blender has an incredible number of users, the pricing is targeted to reach as many Blender users as possible.
• Yes, it's safe. In order to avoid any possible dispute, most assets have been created from scratch. Even all the mocap data were created using our own mocap system. When necessary, purchases have been made to be able to add a few third-party textures to Citigen's texture library.  (Caution: The "Low Poly Building Pack" included in the original Citigen package might include a few facade shaders from Blenderkit.com which may or may not allow its shaders to be included in a distributed package. Though introduced as quick testing purposes, if you want to use the Low Poly Building Pack for commercial purposes, you are advised to subscribe to Blenderkit for a small fee, just to be safe. )

Please be sure to uninstall the existing version before installing the newer version.
(It's probably the same with almost all other Blender add-ons.)
(And please check if there's another addon already installed that limits/controls the installation of other addons such as  "Clean Panel.")