https://discuss.frappe.io/t/unable-to-save-doctype-valueerror-could-not-convert-string-to-float
I check the schema but didnt find anything unusual:
+---------------------+---------------+------+-----+-------------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------------+---------------+------+-----+-------------+-------+
| name | varchar(140) | NO | PRI | NULL | |
| creation | datetime(6) | YES | | NULL | |
| modified | datetime(6) | YES | | NULL | |
+---------------------+---------------+------+-----+-------------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------------+---------------+------+-----+-------------+-------+
| name | varchar(140) | NO | PRI | NULL | |
| creation | datetime(6) | YES | | NULL | |
| modified | datetime(6) | YES | | NULL | |
+---------------------+---------------+------+-----+-------------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------------+---------------+------+-----+-------------+-------+
| name | varchar(140) | NO | PRI | NULL | |
| creation | datetime(6) | YES | | NULL | |
| modified | datetime(6) | YES | | NULL | |
| modified_by | varchar(140) | YES | | NULL | |
| owner | varchar(140) | YES | | NULL | |
| docstatus | int(1) | NO | | 0 | |
| idx | int(8) | NO | | 0 | |
| activity_type | varchar(140) | YES | | NULL | |
| from_time | datetime(6) | YES | | NULL | |
| description | text | YES | | NULL | |
| expected_hours | decimal(21,9) | NO | | 0.000000000 | |
| to_time | datetime(6) | YES | | NULL | |
| hours | decimal(21,9) | NO | | 0.000000000 | |
| completed | int(1) | NO | | 0 | |
| project | varchar(140) | YES | | NULL | |
| project_name | varchar(140) | YES | | NULL | |
| task | varchar(140) | YES | | NULL | |
| is_billable | int(1) | NO | | 0 | |
| sales_invoice | varchar(140) | YES | | NULL | |
| billing_hours | decimal(21,9) | NO | | 0.000000000 | |
| base_billing_rate | decimal(21,9) | NO | | 0.000000000 | |
| base_billing_amount | decimal(21,9) | NO | | 0.000000000 | |
| base_costing_rate | decimal(21,9) | NO | | 0.000000000 | |
| base_costing_amount | decimal(21,9) | NO | | 0.000000000 | |
| billing_rate | decimal(21,9) | NO | | 0.000000000 | |
| billing_amount | decimal(21,9) | NO | | 0.000000000 | |
| costing_rate | decimal(21,9) | NO | | 0.000000000 | |
| costing_amount | decimal(21,9) | NO | | 0.000000000 | |
| parent | varchar(140) | YES | MUL | NULL | |
| parentfield | varchar(140) | YES | | NULL | |
| parenttype | varchar(140) | YES | | NULL | |
+---------------------+---------------+------+-----+-------------+-------+