General help

Maybe there are some null values in the database column mdl_role_assignments.userid. This column should not allow nulls.

Do you know how to run database queries? What do you get when you run (assuming you have MySQL or MariaDB):

DESC mdl_role_assignments;

On my Moodle 3.7 site this is:

+--------------+--------------+------+-----+---------+----------------+ 
| Field        | Type         | Null | Key | Default | Extra          | 
+--------------+--------------+------+-----+---------+----------------+ 
| id           | bigint(10)   | NO   | PRI | NULL    | auto_increment | 
| roleid       | bigint(10)   | NO   | MUL | 0       |                | 
| contextid    | bigint(10)   | NO   | MUL | 0       |                | 
| userid       | bigint(10)   | NO   | MUL | 0       |                | 
| timemodified | bigint(10)   | NO   |     | 0       |                | 
| modifierid   | bigint(10)   | NO   |     | 0       |                | 
| component    | varchar(100) | NO   | MUL |         |                | 
| itemid       | bigint(10)   | NO   |     | 0       |                | 
| sortorder    | bigint(10)   | NO   | MUL | 0       |                | 
+--------------+--------------+------+-----+---------+----------------+ 

Also try:

 SELECT * FROM mdl_role_assignments WHERE userid IS NULL OR userid = 0;

While execute the following query

DESC mdl_role_assignments;

SELECT * FROM mdl_role_assignments WHERE userid IS NULL OR userid = 0;