How to edit the default.aspx page at a Project Online site?

[spoguys]

Hey,

I am pretty sure that I had edited the default.aspx page at https://xxxx.sharepoint.com/sites/pwa/default.aspx last October. However, I realized that I cannot update this default page anymore. I tried to append mode=edit at the end of the URL, but no luck.

Would you please advise how I should update this default.aspx page as a site collection administrator?

Thank you very much!

[#2]

I am pretty surprised that I don't have permission to update the page as a site collection administrator:

Maybe it is some latest changes that Microsoft made recently. 

[#3]

@spoguys  You want to look a change Microsoft implemented to remove custom scripting from OneDrive and SharePoint as it may be related. You will need your Global SharePoint Admin to go M365 Message Central and review Message ID MC714186. Basically, they will need to run a PowerShell script to re-enable custom scripting on affected PWAs.  In our case, we would add new fields to a PDP then save it, but when we revisited the PDP the fields were gone.  Running the script fixed the issue.