HI,
I just installed Linux Mint on my computer. However, I found it impossible to adjust the brightness. When I attempted to change screen resolution(since everything was extremely small), it had only one option. Besides, a message saying 'Check your video drivers' always appeared on the far-right corner. When I launched driver manager, it strangely says 'Your computer does not need any additional drivers'.
I guess it's something wrong with graphic card. I browsed a few threads but couldn't figure out. Any idea what's going on?

Without any information about your graphics card, monitor, or driver we would be blind guessing.
Can you provide any information about the hardware?
Do you know how to find and report the video driver in use?

CPU:
Topology: 8-Core model: AMD Ryzen 7 4800U with Radeon Graphics bits: 64
type: MT MCP arch: Zen rev: 1 L2 cache: 4096 KiB
flags: avx avx2 lm nx pae sse sse2 sse3 sse4_1 sse4_2 sse4a ssse3 svm
bogomips: 57494
Speed: 1397 MHz min/max: 1400/1800 MHz boost: enabled Core speeds (MHz):
1: 1397 2: 1398 3: 1397 4: 1397 5: 1397 6: 1397 7: 1397 8: 1397 9: 1397
10: 1397 11: 1397 12: 1397 13: 1397 14: 1397 15: 1397 16: 1397
Graphics:
Device-1: AMD Renoir vendor: Lenovo driver: N/A bus ID: 03:00.0
chip ID: 1002:1636
Display: x11 server: X.Org 1.20.8 driver: ati,fbdev
unloaded: modesetting,radeon,vesa resolution: 2560x1600~93Hz
OpenGL: renderer: llvmpipe (LLVM 9.0.1 128 bits) v: 3.3 Mesa 20.0.4
compat-v: 3.1 direct render: Yes

Not sure if this is enough.

What Kernel is running on that machine? What version of MINT have you loaded?

The graphics stack and driver that takes advantage of that GPU architecture comes in stable kernel 5.6 and later. I was thinking you should be good there, but do not have a current MINT instance to check on hand.

The VESA module, if that is (as it appears) indeed what you are running, is generally very stable and trustworthy, but uses fewer of the settings and less of the power of your GPU. There may be no better option, but I would look for one to be certain.

So as a first cut: check/update kernel (linux-image) and check for a more appropriate graphics driver.

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Not sure which Kernel. Version is Mint 20, the latest one.
I tried another distribution called manjaro yesterday, and it worked without those problems. It had resolutions I wanted and brightness can be perfectly adjusted.

Manjaro is ARCH based and a bit more cutting edge. I like it.
If you want to get MINT running right on that hardware, I would check what video driver Manjaro uses and adjust Mint to use that one.
I might also do a kernel update on Mint to run a later kernel. Mint is a bit conservative there (for stability) and it may make the difference.

The command to get the kernel information on version is in a terminal.

If you decide to just run Manjaro instead, that is a valid option.

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