right click on grass block and then spawn pebbles

You can use "if is block at x y z tagged in (tag)" condition to make sure that it is a grass block, you can replace (tag) with any tag for a block, I think if you click on one of the buttons at the top of the screen it will pull up a list of common tags, including a grass one.

The solution is easy. create a code with minecraft blocks that does what you want, check it and when it works, we edit the code

how to edit the code: click the left button on the procedure, it lights up but it does not open, then on the left side 4 options under the plus sign to add, we have to edit the code of the selected element, we click on it and open it, then we search The emitter block in this case would be grass (note that grass is not the same as ground with grass) and we change it by hand writing the TF block.
Then we do the same with the receiver, that is, in pebble, we change the name of the minecraft block to that of TF and click save code, you have the option at the top and it would be compatible with TF without any problem.

Sorry I took so long to respond.  you can use the procedure I have written below without needing custom code, I have tested it and it will work.  Different procedure blocks are marked as (block(sub-block) [configurable sub-part])

"(If(Is (get block at (x) (y) (z)) material type [grass])

do     (spawn item/gem (pebble) at (x) (y) (z))"

I realized that there is no grass tag in vanilla minecraft or forge.  I thought I remembered one being there, but I still should have checked.