Monitor looks like it is not VESA compatible. You'll have to use a VESA adapter which holds the monitor using a clamp feature.
Something like this should work
I want to recommend this product at Amazon
VIVO Adapter VESA Mount Kit for 20 to 30 inch LED LCD Monitor Screen, 75mm and 100mm mounting Bracket, Stand-VAD1
by V I V O
Learn more: https://www.amazon.com/dp/B00RTS5WWM/ref=cm\_sw\_em\_r\_mt\_dp\_C701KBMB7FX03MQ44275
Not true Look at the one in the link i posted, or the pictures below on this amazon listing
These are for adding a vesa to non vesa monitor by clamping to the outside corners kind of like a giant cel phone tripod or car mount. I know its not a diy answer but seems like the best possible solution given the modest cost ($15 on amz)
Those mounting holes in the back of the monitor are called a VESA mount. Unfortunately it looks like your monitor doesn’t have them.
There are a number of adapters you can use like this. I’m not sure if this exact mount will work, but something like that.
Personally I would buy a new monitor that includes VESA mounting holes, but mostly for aesthetic reasons.
There are products like this. I have not used this one or any thing like this. VIVO Adapter VESA Mount Kit for 20" to 30" LED LCD Monitor Screen, 75mm and 100mm mounting bracket (STAND-VAD1) https://www.amazon.com/dp/B00RTS5WWM/ref=cm_sw_r_other_awd_siC5wbSQT1E3E
If it doesn't have VESA compatibility or a removable stand (which I don't think it has either) this is a nice second option for mounting: https://www.amazon.com/dp/B00RTS5WWM/ref=cm_sw_r_other_awd_PLHXwb46889CR I can't claim to have used these myself but I've seen a friend of to that has and they never had a problem. So there's one option!
I got a monitor that didn't support vesa mounting and I purchased this. I'd definitely recommend, however it took a little bit of time finding the best position for the four arms so that none of them blocked the connections to the back of the monitor. http://www.amazon.com/VIVO-Adapter-Monitor-mounting-STAND-VAD1/dp/B00RTS5WWM/ref=sr_1_2?s=office-products&ie=UTF8&qid=1449687960&sr=1-2&keywords=vivo+adapter+vesa+mount+kit
You could get an adapter for the mount. its not as sleek looking but it does the job https://www.amazon.com/VIVO-Adapter-Monitor-mounting-Stand-VAD1/dp/B00RTS5WWM/
Seems like somehow all 9 screws are toast. You'd be best to get one of those VESA mounts that hugs the edges of the display, something like this https://www.amazon.com/VIVO-Adapter-Monitor-mounting-Stand-VAD1/dp/B00RTS5WWM/ plus a monitor stand or monitor arm like this https://www.amazon.com/MOUNTUP-Single-Monitor-Mount-Stand/dp/B087LSHXJH/
I've been able to identify a part number, 13PT02J0M24011 but naturally no one has any idea where to get it. certainly not from the Asus online store. I used the "contact the CEO" link and the person I've been conversing with seems to be struggling with the concept.
I was able to use this:
https://www.amazon.ca/gp/product/B00RTS5WWM/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
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by drilling some holes in the back plastic cover and screwing it into the mounting points that the stand (and the unavailable adapter) connect to. It's not ideal but it is fairly solid. LMK if you want a picture of how I rigged it up, it might help you. Asus certainly won't.
Eg. https://www.amazon.com/VIVO-Adapter-Monitor-mounting-Stand-VAD1/dp/B00RTS5WWM/ref=asc_df_B00RTS5WWM/
Just slap a vesa mount adapter on it...
The portrait monitor is some cheap Acer monitor that I got on clearance at microcenter, so it doesn't have a vesa mount. So, I bought this vesa adapter, and attached it to my monitor arm: https://www.amazon.com/VIVO-Adapter-Monitor-mounting-Stand-VAD1/dp/B00RTS5WWM/ref=sr_1_6?dchild=1&keywords=vesa+mount+adapter&qid=1617465529&s=electronics&sr=1-6
I bought a different VESA mount - https://www.amazon.com/dp/B00RTS5WWM?ref=ppx_pop_mob_ap_share - per advice one of the folks in Dell Canvas 27 subreddit. Not the best solution but good enough. Can't imagine having it in the default position, too.
I think this could work for you! https://www.amazon.com/dp/B00RTS5WWM/ref=cm_sw_r_cp_apa_S-XGBbBS3XY69
Uhhhh, third time's the charm?