#pixel $PIXEL

It sounds like you want a clearer or more polished version of that line. Here are a few improved options depending on tone:

Simple & clear:

“This can only be completed once and will then be marked as complete.”

Slightly more formal:

“This action can be completed only once, after which it will be marked as completed.”

Instructional tone:

“You can complete this only once. Once done, it will be marked as completed.”

UI-friendly (concise):

“One-time action — marked complete after submission.”

If you tell me where you plan to use it (app, form, error message, etc.), I can tailor it more precisely.