I want to make sure I am clear on what you are asking as these are two different things
Not able to sign in <== if you mean specifically into the power up program that you already have been approved for, you would have been sent an email explaining how to sign in and with your new credentials for the PowerUp Program Environment.
or
register to Power Up Program <== this is different as you do not "sign in" to register. What I mean is, the page itself is not required to be signed into anything as you are providing your email etc in the sign up process.
So are you having issues signing into the environment itself (as in the Power Up Program you have been assigned too)
or are you having a problem registering for a Cohort?
Make sure that you are signing into the Program with an incognito, private browser instance and using the credentials you were associated too in the program.
to get support
If these suggestions help resolve your issue, Please consider Marking the answer as such and also maybe a like.
Thank you!
Sincerely, Michael Gernaey