Please consider clear cache of your browser, then sign in PowerApps again, try your operation again, check if the issue is solved.
Also please check if the PowerApps License of your current sign in account has been expired,if true, please purchase another license for your sign in account.
Note: Currently, within PowerApps, if you want to access CDS functionality, you must own PowerApps Plan 1 license. If you are using a PowerApps Plan 2 free trial license for your account, please check if the free trial license has been expired.
If you have enough permission to the tblClients Entity within CDS of a PowerApps Environment, but still have issue with accessing the tblClients Entity within a PowerApps app, please consider submit an assisted support ticket through the following link: