calculated field in SPO

first, sorry when I am wrong with answering my own question. But I don't know how to add Screenshots within a comment to an answer...

So here are the screenshots of what I tried to built with your help. What do I need to fill into the add-function instead of "Apply_to_each"? "Leistungserbringung gesamt Z" is not working here.

Just trying to get this right:

1.) The Flow will always be triggered when a new item in my list "Tagesdokumentation_TEST" will be created.

2.) Always the variable "gesamt_IST" will be initialized (does that mean the variable will be ready to calculate with?)

3.) Gets all items with the same value in column "KlientInY", so the flow can use it in the next steps. (In my list "KlientInY" is a lookup column from another list)

4.) Compose-function should define that all times (in column "Leistungserbringung gesamt Z") with the same value for KlientInY should be summed up in column "Gesamt IST"???

5.) The variable is defined in step 4 but now the output of step 4 is used to set the variable?

Calculated column is not able to do your work. In SharePoint Online I will suggest using MS Power Automate(used to called as Flow) to do so. I build a test list and MS flow to get the column sum with condition.

My list:

My flow:

The formula:add(variables('Total Time'),items('Apply_to_each')?['SpentHour'])

After this, we have the calculated sum value. It is stored in the variable Total Time. You can use it to update the list you need.

Update

If we need to filter on a lookup column, we need to change the filter query as

<Field Internal Name>/Title eq <Field Internal Name> value

Like this: