WordPress is very definitive sometimes! When I ran into this problem of logging into the /wp-admin/ page only to be told “Sorry You are not allowed to access this page”, I found that most people online that were complaining were people who were already administrators on the site.
Step 1 is to disable all your plugins via FTP/SFTP, and try to log in again.
If this fails, disable your themes, again using FTP/SFTP and try to log in again.
The final step to try, could come from having added yourself manually to the database or a change in the database table prefix.
Go to your table
And find your user_id’s corresponding fields for what is probably currently wp_capabilities.
Change wp_capabilities to read
Likewise, change your wp_user_level to
When you return to log in, at this stage you should find that you are able to log in successfully.