If i want to manually create my partitions, and encrypt my linux installation, can i create only one partition for root and install the whole linux system on that partition, or it's necessary to create a separate partition for boot?

This is Linux - anything is possible. Including what you propose.
Depending on distro it may develop into an extremely educational exercise.

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As far as I know, you only need to encrypt your own personal data, so having a separate /home would be the way to go.
(Keep a copy of your encryption key separately as a backup in case of any problems.)

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I don't agree with that, but my question is more about will i have any issues if i install linux only on one partition.

I did that for years and did not have any problems - so long as there was proper monitoring and alerting in place.
For example, if there is an issue with filesystem filling up due to logs or application or whatever, that can create problems.
If for any reason you want to install different flavor (or upgrade with fresh install), then creating separate partition for data could be easier.
And for NIST compliance, creating separate partition is necessary - if you have to deal with this types of things.

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According to specification uefi partition should be separated as it is formatted as fat-12, fat-16 or fat-32

Unless you want your distro to run from fat partition?

If you are not planning to install efi/uefi, then if you set distro installer to automatic partitioning, you will end up in a single partition with home/user folder.

To get back to the original question, a separate boot partition is not a must. If a distro includes an automatic partitioner, in my experience it normally creates a single /root partition and puts everything into that, unless you choose a "manual" or "custom" option.

I normally create a separate / and /home, so, in case I have to reinstall, the data in /home gets preserved. (Yes, I back it up to external medial first if I'm reinstalling--or perhaps even changing distros--but, if all goes well, I don't need the back up.)

In the early days, when hard drives were very small (think less than 40MB), it was not unusual to also have additional separate partitions for /tmp, /var, /sbin, and perhaps other directories.

This article from the Arch wiki give a good overview of Linux partitioning: https://wiki.archlinux.org/index.php/Partitioning

Ask yourself this. How do you plan to decrypt the boot sector before it boots?

Here is a tutorial to do it with ubuntu. Never tried it so can't vouch for it.

I am gonna go out on a limb here and express an opinion. Based on the fact that the /boot directory contains the kernel images, I would not put /boot on a separate partition. My thought is that /boot on a separate partition 'could' affect booting performance (although I could be wrong). I ended up making /usr and /home on separate partitions for space reasons on my most recent install of PCLinuxOS2020. Just feel possible unattended issues 'might' leave one unhappy with a /boot on a separate partition.

You can use /boot with encryption but it's not simple and there's no guarantee that the method used will continue to work as grub changes: encrypted /boot
The obvious question is why you feel the need to do it? It's also a good idea to put /home on a separate partition, so that a clean installation is possible without having to restore all your data.

Have to agree with Fatmac: if you have problems with your installation you can get into a state where you can't access your data if you use whole disk encryption. Been there, done that.

What data would one not cover by using an encrypted /home if that is where you store your data?