Cannot delete photos from iPad as trash button is grayed out

As Servant of Cats suggested, pictures that are transferred through a cable from a computer, perhaps with iTunes, can be shown by Photos, but they're not "owned" by Photos. (I think that I was the first one to use that metaphor!) So these pictures can't be edited or deleted by Photos, and they can't be synchronized with iCloud Photos. The idea is that the originals are kept on the computer, and they are transferred to the phone temporarily.

You have found one way to delete them -- turn on iCloud Photos! Another way is to use the computer to transfer an empty set of "pictures" to the phone, and the empty set will replace the others. In using iTunes for this, it may give the option to deselect the pictures that are already there. On a Mac we might do it with the app Image Capture in the Applications folder. This is really just a holdover from an earlier era.

It would help if you could give us some more information:

1. What model is your iPad? What iPadOS does it use?
2. Does this apply to all your pictures on the iPad?
3. Were these pictures taken on your iPad, or were they imported from another source, like from a web site or transferred from a computer?
4. Can you Edit these pictures?
5. Are you using iCloud Photos to synchronize your devices? If so
6. …Do you have “Optimize Storage” checked in Settings>[name]>iCloud>Photos>
7. …Do you see all your iPad pictures at iCloud.com?
8. …Can you delete the pictures at iCloud.com?
9. …What is your storage in iCloud? At Settings>[name]>iCloud>Storage, mine says 166.6 GB of 200 GB used. What does yours say? 
10. What is your storage on this iPad? At Settings>General>iPad Storage, mine says 215 GB of 512 GB used. What does yours say? 
11. At Settings>Battery, what do Battery Health, Maximum Capacity, and Cycle Count say?a
12. Has this problem always existed, or did it just start? Can you think of anything that changed that might affect your device?

By any chance did you originally load those photos onto the iPad from a computer using "manual" synchronization over a cable (e.g., from iTunes or the Finder)?

I've seen some threads in these forums that suggest that the iPhone/iPad versions of Photos remember that such photos are "owned by" the computer and treat them differently than others. (Although I've never experimented to see what the situation is.)

Thanks Richard,

M4 running OS27 I only had 9 photos on the iPad. So old that I really can't remember where they came from.

I was already synching all my photos on my iCloud, iMAC, my MACBOOKPRO and my iphone. So the path of least resistance was simply to start including the iPAD photos as well.

It worked fine , however, I lost the original 9 photos from the iPad. I thought it would have "merged" them with the ones in iCloud but not. However, no great loss.

Thanks again , Richard.