Learn Dutch: Omdat het hele deeg nog niet plakkerig is, voeg ik water toe. - Because the whole dough is not yet sticky, I am adding water.

Both make sense, but they have a slightly different focus. Het hele deeg focuses on the dough as a single, continuous entity (the entire batch in front of you). Al het deeg focuses more on the total quantity or amount. When talking about the physical state of a singular mass or object, het hele is usually the more natural choice for expressing "the whole of it."

Deeg is an het word. Why does the adjective heel get an -e ending to become hele?

Adjectives in front of het words only drop the -e ending when they are indefinite—meaning they follow een, geen, zo'n, or no article at all. Because we are using the definite article het here, the adjective follows the standard rule and takes the -e ending: het hele deeg.

Why does the second part of the sentence start with the verb voeg instead of ik voeg?

This is due to the V2 (verb-second) rule. The entire first clause (Omdat het hele deeg nog niet plakkerig is) acts as the first position of the main sentence. To keep the main verb in the second position, voeg must come immediately after the comma, which inverts the subject and verb, pushing ik to the third position.