"I will check this."

Translation: Я проверю это.

[Mutusen]

"Я это проверю" was not accepted, is this order wrong?

[deactivated user]

It's not wrong. According to Rosenthal’s style guide (§ 180.1), both are equally correct. You should report it next time you get this sentence.

[Scian4]

Accepted November 2020

[Ivorah]

Why isn't it я этого проверю