Script to take screen shots every 30 sec

Launch 'Terminal' and enter ...

man screencapture

... and press the <return> key; or,

screencapture -h

... and press the <return> key.

Once you have decided how to utilize 'screencapture' place ...

do shell script "screencapture ..."

... within your specific AppleScript code - which calls 'screencapture' every n seconds over period y (where 'x' and 'y' are specific to your needs).

In your code, 'tell application "Finder"' and its 'end tell' are not needed, no file name is provided; and, nor is a file format for '-t' provided.

Code sample -

set dFolder to "~/Desktop/screencapture/"

do shell script ("mkdir -p " & dFolder)

repeat with i from 1 to 900 -- Repeat 900 times.
do shell script ("screencapture -c " & dFolder & (i as string) & ".png") -- Where '-c' copies the screen shot to the clipboard.
delay 30 -- Wait for 30 seconds.
end repeat

Entering ...

mac unix +screencapture

... into a search engine presents many results.

set dFolder to "~/Desktop/screencapture/"

do shell script ("mkdir -p " & dFolder)

repeat 900 times -- Repeat 900 times.
set tTime to do shell script "date +%H%M%S"
do shell script ("screencapture " & dFolder & tTime & ".png") -- Capture screen.
delay 30 -- Wait for 30 seconds.
end repeat

To include the date, as well ...

set dFolder to "~/Desktop/screencapture/"

do shell script ("mkdir -p " & dFolder)

repeat 900 times -- Repeat 900 times.
set dateTime to do shell script "date +%Y%m%d_%H%M%S"
do shell script ("screencapture " & dFolder & dateTime & ".png") -- Capture screen.
delay 30 -- Wait for 30 seconds.
end repeat