The presenter was discussing Kate's photo-sharing campaign, named Hold Still, which ends today (June 18).
The Duchess has asked the public to share their own pictures to show their lockdown experiences.
As Holly tried to tell viewers where they could send their pictures in to, she got the website wrong.
Her co-star Phillip Schofield told viewers: "The deadline for photos submissions is actually today and it's at 6pm."
Holly continued: "Yes, to upload your photos head to NPG.org.uk forward slash hold..." as she slipped up on her words.
She said: "What did I just say there? I even practised this and I got it wrong! Let's do it one more time shall we?
"Here we go, head to NPG.org.uk forward slash hold still."
However, it was wrong again and Phil jokingly called her out.
He said: "Hold dash still," to which Holly said: "There you go, we'll put it on the screen. Read it yourselves it's quite a complicated one."
This is my actual job to read stuff and I can't do it sometimes.
Phil burst into hysterics and covered his face with his hands.
Holly said: "This is my actual job to read stuff and I can't do it sometimes.
"I didn't believe there was a final dash between 'hold still'. Unnecessary I think."
Phil quipped: "Shall I tell the Duchess of Cambridge or shall you?"
Laughing, Holly said: "Sorry ma'am."
Kate teamed up with the National Portrait Gallery for her photo project named Hold Still.
The exhibition will feature 100 photographs, giving a glimpse into the nation's experience of lockdown.
Speaking about the project, Kate said: "We’ve all been struck by some of the incredible images we’ve seen."
In addition, she said: "Some desperately sad images showing the human tragedy of this pandemic.
"Other uplifting pictures showing people coming together to support those more vulnerable."
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