Why is Tesseract unable to detect the single digit in that image?

Improving the quality of the output is your "holy scripture" when working with Tesseract. Especially, the page segmentation method should always be explicitly set. Here (as most of the times), I'd opt for --psm 6:

Assume a single uniform block of text.

Even without further preprocessing of your image, you already get the desired result:

import cv2
import pytesseract

image = cv2.imread('gBrcd.png')
text = pytesseract.image_to_string(image, config='--psm 6')
print(text.replace('\f', ''))
# 1
# LOW: 56

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System information
----------------------------------------
Platform:      Windows-10-10.0.19041-SP0
Python:        3.9.1
PyCharm:       2021.1.1
OpenCV:        4.5.2
pytesseract:   5.0.0-alpha.20201127
----------------------------------------