Well, I had following the discussion out there:
https://help.forumotion.com/t158838-forum-sitemap#1105141

But it's not works even If I added extra slash

even Google has changes the style of webmaster and there is not full path to add only the Sitemap name as you can see:

hello

please do not open similar threads to multiple support forums

since you have already opened a similar thread to the Arabic support and your forum is Arabic this thread will be closed

https://help.ahlamontada.com/t1095170-topic

Hello,

The problem is still active and unfortunately comes from Google. We have tried many solutions without success so far... The sitemaps of our forums are correct. We opened a topic on the support forum dedicated to the Search Console service: https://support.google.com/webmasters/thread/57626536?hl=en&msgid=57626536.

Here is the answer we received:

Yes, "Sitemap could not be read". is just the 'long' form of "Couldn't fetch".

Really it should IMHO be "Sitemap has not yet been read" or even just "Pending/Unknown"

... as as not to imply there is a technical issue.