@irocha try following solution, it should work, as i have already tested it:
1. Add compose action with following expressions, here in the expression first replace outputs('Get_response_details')?['body/r11e08973b90a4c588aeaa2a4ebe1be37'] with your form checkbox output dynamic content.
split(outputs('Get_response_details')?['body/r11e08973b90a4c588aeaa2a4ebe1be37'],',')
2. Add Initialize Variable action of type Array and name as ChoiceValues
3. Add Apply to each action and select output from the Compose action which we added in step 1
4. Inside apply to each action, add Compose action with following expression:
replace(replace(replace(items('Apply_to_each'),'"',''),'[',''),']','')
5. Under compose action add Append to array variable action and configure it as shown in below screenshot, here you will use output of compose action added in step 4

6. After apply to each action, outside of it, add Update Item action and select the ChoiceValues variable for dynamic content of your Choice column in SharePoint list

Here is the full flow screenshot
