"I am sewing a red dress."
Translation:Mi kudras ruĝan robon.
"I am <do>ing" should be translated as "I <do>" in Esperanto, so "estas" shouldn't be used there
To be more clear, that's because "to be <verb>ing" is present-progressive, which english often uses where other languages use plain present tense ("to <verb>”.
In languages (like esperanto) that make this distinction, usually one should use the plain present tense. However, when emphasizing that something is occurring right this moment, instead of generically currently happening (usually something the speaker is actively doing, or a shorter-term event), one could use the present-progressive, which is not formed "estas <root>as".
The correct way to form present progressive in esperanto is "estas <root>ant<o/a>”